diff --git a/website/server/controllers/api-v3/groups.js b/website/server/controllers/api-v3/groups.js
index f00e90f497..ed3434c158 100644
--- a/website/server/controllers/api-v3/groups.js
+++ b/website/server/controllers/api-v3/groups.js
@@ -603,7 +603,14 @@ api.joinGroup = {
       group.leader = user._id; // If new user is only member -> set as leader
     }
 
-    group.memberCount += 1;
+    if (group.type === 'party') {
+      // For parties we count the number of members from the database to get the correct value.
+      // See #12275 on why this is necessary and only done for parties.
+      const currentMembers = await group.getMemberCount();
+      group.memberCount = currentMembers + 1;
+    } else {
+      group.memberCount += 1;
+    }
 
     let promises = [group.save(), user.save()];
 
@@ -948,7 +955,14 @@ api.removeGroupMember = {
     }
 
     if (isInGroup) {
-      group.memberCount -= 1;
+      // For parties we count the number of members from the database to get the correct value.
+      // See #12275 on why this is necessary and only done for parties.
+      if (group.type === 'party') {
+        const currentMembers = await group.getMemberCount();
+        group.memberCount = currentMembers - 1;
+      } else {
+        group.memberCount -= 1;
+      }
 
       if (group.quest && group.quest.leader === member._id) {
         group.quest.key = undefined;
diff --git a/website/server/models/group.js b/website/server/models/group.js
index 237c6f6989..996be822a0 100644
--- a/website/server/models/group.js
+++ b/website/server/models/group.js
@@ -1415,23 +1415,12 @@ schema.methods.leave = async function leaveGroup (user, keep = 'keep-all', keepC
     } else {
       members = await User.find({ 'party._id': group._id }).select('_id').exec();
     }
-
     _.remove(members, { _id: user._id });
 
     if (members.length === 0) {
       promises.push(group.remove());
       return Promise.all(promises);
     }
-  // otherwise If the leader is leaving
-  // (or if the leader previously left, and this wasn't accounted for)
-  } else if (group.leader === user._id) {
-    const query = group.type === 'party' ? { 'party._id': group._id } : { guilds: group._id };
-    query._id = { $ne: user._id };
-    const seniorMember = await User.findOne(query).select('_id').exec();
-
-    // could be missing in case of public guild (that can have 0 members)
-    // with 1 member who is leaving
-    if (seniorMember) update.$set = { leader: seniorMember._id };
   }
   // otherwise If the leader is leaving
   // (or if the leader previously left, and this wasn't accounted for)
@@ -1720,13 +1709,7 @@ schema.methods.hasCancelled = function hasCancelled () {
 
 schema.methods.updateGroupPlan = async function updateGroupPlan (removingMember) {
   // Recheck the group plan count
-  let members;
-  if (this.type === 'guild') {
-    members = await User.find({ guilds: this._id }).select('_id').exec();
-  } else {
-    members = await User.find({ 'party._id': this._id }).select('_id').exec();
-  }
-  this.memberCount = members.length;
+  this.memberCount = await this.getMemberCount();
 
   if (this.purchased.plan.paymentMethod === stripePayments.constants.PAYMENT_METHOD) {
     await stripePayments.chargeForAdditionalGroupMember(this);