diff --git a/backend/src/routes/schedule.js b/backend/src/routes/schedule.js
index cc0516e..88cc666 100644
--- a/backend/src/routes/schedule.js
+++ b/backend/src/routes/schedule.js
@@ -308,6 +308,15 @@ router.patch('/:id', authMiddleware, async (req, res) => {
           const member = await queryOne(req.schema, 'SELECT 1 FROM user_group_members WHERE user_id=$1 AND user_group_id=$2', [req.user.id, ugId]);
           if (!member) return res.status(403).json({ error: 'You can only assign groups you belong to' });
         }
+        // Preserve any existing groups on this event that the user doesn't belong to
+        // (e.g. groups added by an admin) — silently merge them back into the submitted list
+        const existingGroupIds = (await query(req.schema, 'SELECT user_group_id FROM event_user_groups WHERE event_id=$1', [req.params.id])).map(r => r.user_group_id);
+        const submittedSet = new Set(userGroupIds.map(Number));
+        for (const gid of existingGroupIds) {
+          if (submittedSet.has(gid)) continue;
+          const member = await queryOne(req.schema, 'SELECT 1 FROM user_group_members WHERE user_id=$1 AND user_group_id=$2', [req.user.id, gid]);
+          if (!member) userGroupIds.push(gid);
+        }
       }
     }
     const fields = { title, eventTypeId, startAt, endAt, allDay, location, description, isPublic, trackAvailability, recurrenceRule, origEvent: event };
diff --git a/frontend/src/components/SchedulePage.jsx b/frontend/src/components/SchedulePage.jsx
index a264b37..f05b171 100644
--- a/frontend/src/components/SchedulePage.jsx
+++ b/frontend/src/components/SchedulePage.jsx
@@ -555,7 +555,8 @@ function EventForm({ event, userGroups, eventTypes, selectedDate, onSave, onCanc
   const [desc,setDesc]=useState(event?.description||'');
   const [pub,setPub]=useState(event?!!event.is_public:!!isToolManager);
   const [track,setTrack]=useState(!!event?.track_availability);
-  const [grps,setGrps]=useState(new Set((event?.user_groups||[]).map(g=>g.id)));
+  const accessibleGroupIds = new Set(userGroups.map(g=>g.id));
+  const [grps,setGrps]=useState(new Set((event?.user_groups||[]).map(g=>g.id).filter(id=>isToolManager||accessibleGroupIds.has(id))));
   const [saving,setSaving]=useState(false);
   const [showTypeForm,setShowTypeForm]=useState(false);
   const [localTypes,setLocalTypes]=useState(eventTypes);