diff --git a/assets/js/hooks.js b/assets/js/hooks.js
index 9cacc1f..1f20179 100644
--- a/assets/js/hooks.js
+++ b/assets/js/hooks.js
@@ -207,6 +207,7 @@ export const Hooks = {
   DateTimePicker: {
     mounted() {
       this.hiddenInput = this.el.querySelector("[data-datetime-value]")
+      this.unusedInput = this.el.querySelector("[data-datetime-unused]")
       this.dateInput = this.el.querySelector("[data-datetime-date]")
       this.timeInput = this.el.querySelector("[data-datetime-time]")
       this.timezoneLabel = this.el.querySelector("[data-datetime-timezone]")
@@ -264,6 +265,11 @@ export const Hooks = {
         this.hiddenInput.dispatchEvent(new Event("input", {bubbles: true}))
       }
 
+      this.syncUserInput = () => {
+        if (this.unusedInput) this.unusedInput.disabled = true
+        this.syncHidden()
+      }
+
       this.setQuickTime = event => {
         const minutes = Number(event.currentTarget.dataset.datetimeOffset)
         if (!Number.isFinite(minutes)) return
@@ -271,11 +277,11 @@ export const Hooks = {
         const date = new Date(Date.now() + minutes * 60_000)
         date.setSeconds(0, 0)
         this.applyLocalDate(date)
-        this.syncHidden()
+        this.syncUserInput()
       }
 
-      this.dateInput.addEventListener("input", this.syncHidden)
-      this.timeInput.addEventListener("input", this.syncHidden)
+      this.dateInput.addEventListener("input", this.syncUserInput)
+      this.timeInput.addEventListener("input", this.syncUserInput)
       this.quickButtons.forEach(button => button.addEventListener("click", this.setQuickTime))
 
       const initial = this.parseUtc(this.hiddenInput.value)
@@ -288,8 +294,8 @@ export const Hooks = {
       }
     },
     destroyed() {
-      this.dateInput?.removeEventListener("input", this.syncHidden)
-      this.timeInput?.removeEventListener("input", this.syncHidden)
+      this.dateInput?.removeEventListener("input", this.syncUserInput)
+      this.timeInput?.removeEventListener("input", this.syncUserInput)
       this.quickButtons?.forEach(button => button.removeEventListener("click", this.setQuickTime))
     }
   },
diff --git a/lib/who_need_help_web/components/core_components.ex b/lib/who_need_help_web/components/core_components.ex
index fcf520c..f5f19cf 100644
--- a/lib/who_need_help_web/components/core_components.ex
+++ b/lib/who_need_help_web/components/core_components.ex
@@ -394,6 +394,7 @@ defmodule WhoNeedHelpWeb.CoreComponents do
       assigns
       |> assign(:id, field.id)
       |> assign(:name, field.name)
+      |> assign(:unused_name, unused_input_name(field.name))
       |> assign(:value, normalized_input_value("datetime-local", safe_form_value(field.value)))
       |> assign(:errors, Enum.map(errors, &translate_error(&1)))
 
@@ -418,6 +419,7 @@ defmodule WhoNeedHelpWeb.CoreComponents do
           value={@value}
           data-datetime-value
         />
+        <input type="hidden" name={@unused_name} value="" data-datetime-unused />
         <div class="grid gap-3 sm:grid-cols-2">
           <label>
             <span class="label mb-1 text-xs">{gettext("Date")}</span>
@@ -464,6 +466,13 @@ defmodule WhoNeedHelpWeb.CoreComponents do
     """
   end
 
+  defp unused_input_name(name) do
+    case Regex.run(~r/^(.*)\[([^\[\]]+)\]$/, name, capture: :all_but_first) do
+      [prefix, field] -> "#{prefix}[_unused_#{field}]"
+      _other -> "_unused_#{name}"
+    end
+  end
+
   # Helper used by inputs to generate form errors
   defp error(assigns) do
     ~H"""
diff --git a/lib/who_need_help_web/live/activity_live/show.ex b/lib/who_need_help_web/live/activity_live/show.ex
index b0c7eb0..41bd7ae 100644
--- a/lib/who_need_help_web/live/activity_live/show.ex
+++ b/lib/who_need_help_web/live/activity_live/show.ex
@@ -492,6 +492,14 @@ defmodule WhoNeedHelpWeb.ActivityLive.Show do
                 <p class="mt-1 whitespace-pre-wrap text-sm">{message.body}</p>
               </article>
             </div>
+            <div
+              :if={@activity.status != :open}
+              class="mt-4 rounded-2xl border border-base-300 bg-base-200 p-4 text-sm text-base-content/65"
+            >
+              {gettext(
+                "This activity is closed. The group chat remains available as read-only history."
+              )}
+            </div>
             <.form
               :if={@activity.status == :open}
               for={@message_form}
@@ -548,7 +556,9 @@ defmodule WhoNeedHelpWeb.ActivityLive.Show do
               phx-click="join"
               class="btn btn-info w-full"
             >
-              {gettext("Request to join")}
+              {if @viewer_participant,
+                do: gettext("Request to join again"),
+                else: gettext("Request to join")}
             </button>
             <div :if={@viewer_participant && @viewer_participant.status == :requested}>
               <span class="badge badge-warning">{gettext("Approval pending")}</span>
@@ -556,6 +566,28 @@ defmodule WhoNeedHelpWeb.ActivityLive.Show do
                 {gettext("The organizer has not approved your request yet.")}
               </p>
             </div>
+            <div
+              :if={@viewer_participant && @viewer_participant.status == :left}
+              class="mt-3 rounded-xl border border-base-300 bg-base-200 p-3"
+            >
+              <p class="font-semibold">{gettext("You left this activity")}</p>
+              <p class="mt-1 text-sm text-base-content/60">
+                {gettext(
+                  "You are no longer a participant and cannot access the group chat or exact meeting point. You can request to join again while registration is open."
+                )}
+              </p>
+            </div>
+            <div
+              :if={@viewer_participant && @viewer_participant.status == :declined}
+              class="mt-3 rounded-xl border border-base-300 bg-base-200 p-3"
+            >
+              <p class="font-semibold">{gettext("Your join request was declined")}</p>
+              <p class="mt-1 text-sm text-base-content/60">
+                {gettext(
+                  "You are not a participant. You can send a new request while registration is open."
+                )}
+              </p>
+            </div>
             <button
               :if={
                 @activity.status == :open && @viewer_participant &&
diff --git a/test/who_need_help/activities_test.exs b/test/who_need_help/activities_test.exs
index b29a5d4..b669f7f 100644
--- a/test/who_need_help/activities_test.exs
+++ b/test/who_need_help/activities_test.exs
@@ -368,6 +368,45 @@ defmodule WhoNeedHelp.ActivitiesTest do
     assert request.status == :requested
   end
 
+  test "leaving revokes private activity access and rejoining returns to approval pending",
+       context do
+    {:ok, activity} = Activities.create_activity(context.organizer_scope, context.attrs)
+    {:ok, request} = Activities.request_to_join(context.participant_scope, activity.id)
+    {:ok, _approved} = Activities.approve_participant(context.organizer_scope, request.id)
+    {:ok, approved_activity} = Activities.get_activity(context.participant_scope, activity.id)
+
+    assert Activities.coordinates_for(context.participant_scope, approved_activity).exact
+
+    assert {:ok, _message} =
+             Activities.send_message(context.participant_scope, activity.id, %{
+               "body" => "Visible before leaving"
+             })
+
+    assert {:ok, left} = Activities.leave_activity(context.participant_scope, activity.id)
+    assert left.status == :left
+
+    assert {:ok, left_activity} = Activities.get_activity(context.participant_scope, activity.id)
+    assert Activities.coordinates_for(context.participant_scope, left_activity).exact == false
+    assert Activities.paginate_messages(context.participant_scope, left_activity).entries == []
+
+    assert {:error, :forbidden} =
+             Activities.send_message(context.participant_scope, activity.id, %{
+               "body" => "Must not be visible after leaving"
+             })
+
+    assert {:ok, rejoined} =
+             Activities.request_to_join(context.participant_scope, activity.id)
+
+    assert rejoined.id == request.id
+    assert rejoined.status == :requested
+    assert is_nil(rejoined.reviewed_at)
+    assert is_nil(rejoined.left_at)
+
+    assert Enum.map(Activities.list_my_activities(context.participant_scope), & &1.id) == [
+             activity.id
+           ]
+  end
+
   test "my activities exclude organizers blocked in either direction", context do
     {:ok, activity} = Activities.create_activity(context.organizer_scope, context.attrs)
     {:ok, _request} = Activities.request_to_join(context.participant_scope, activity.id)
diff --git a/test/who_need_help_web/live/mutual_aid_live_test.exs b/test/who_need_help_web/live/mutual_aid_live_test.exs
index 8d3e1af..9c5e481 100644
--- a/test/who_need_help_web/live/mutual_aid_live_test.exs
+++ b/test/who_need_help_web/live/mutual_aid_live_test.exs
@@ -276,6 +276,7 @@ defmodule WhoNeedHelpWeb.MutualAidLiveTest do
            )
 
     assert has_element?(view, "#help_request_expires_at[type='hidden']")
+    assert has_element?(view, "input[name='help_request[_unused_expires_at]']")
     assert has_element?(view, "#help_request_expires_at-date[type='date']")
     assert has_element?(view, "#help_request_expires_at-time[type='time']")
     assert has_element?(view, "#help_request_expires_at-picker[phx-hook='DateTimePicker']")
@@ -319,13 +320,22 @@ defmodule WhoNeedHelpWeb.MutualAidLiveTest do
 
     html =
       render_change(view, "validate", %{
-        "activity" => %{"category_id" => hiking.id}
+        "activity" => %{
+          "category_id" => hiking.id,
+          "starts_at" => "",
+          "_unused_starts_at" => "",
+          "join_deadline" => "",
+          "_unused_join_deadline" => ""
+        }
       })
 
     assert html =~ "Route difficulty"
     assert html =~ "activity[structured_data][route_plan]"
     assert html =~ "activity[structured_data][safety_plan_confirmed]"
     assert html =~ "approve participants individually"
+    refute html =~ "can't be blank"
+    assert has_element?(view, "input[name='activity[_unused_starts_at]']")
+    assert has_element?(view, "input[name='activity[_unused_join_deadline]']")
 
     html =
       render_change(view, "validate", %{
@@ -424,6 +434,33 @@ defmodule WhoNeedHelpWeb.MutualAidLiveTest do
     assert_push_event(participant_view, "reset-message-form", %{id: "activity-message-form"})
     assert render(organizer_view) =~ "I will be there."
 
+    participant_view
+    |> element("button[phx-click='leave']")
+    |> render_click()
+
+    participant_html = render(participant_view)
+    assert participant_html =~ "You left this activity"
+    assert participant_html =~ "You are no longer a participant"
+    assert participant_html =~ "Request to join again"
+    refute participant_html =~ "Approved group chat"
+    refute participant_html =~ "I will be there."
+    refute has_element?(participant_view, "#activity-message-form")
+
+    participant_view
+    |> element("button[phx-click='join']")
+    |> render_click()
+
+    assert render(participant_view) =~ "Approval pending"
+    assert render(organizer_view) =~ "Activity participant"
+
+    organizer_view
+    |> element("button[phx-click='approve']")
+    |> render_click()
+
+    participant_html = render(participant_view)
+    assert participant_html =~ "Approved group chat"
+    assert participant_html =~ "I will be there."
+
     participant_view
     |> form("#activity-report-form",
       report: %{reason: "harassment", details: "Review this activity context."}
@@ -466,6 +503,8 @@ defmodule WhoNeedHelpWeb.MutualAidLiveTest do
 
     assert render(participant_view) =~ "Completed"
     refute has_element?(participant_view, "button[phx-click='leave']")
+    refute has_element?(participant_view, "#activity-message-form")
+    assert render(participant_view) =~ "read-only history"
 
     stop_live_view(organizer_view)
     stop_live_view(participant_view)