diff --git a/src/Handler/Submission.hs b/src/Handler/Submission.hs
index 9f3fe2788..2148f3a28 100644
--- a/src/Handler/Submission.hs
+++ b/src/Handler/Submission.hs
@@ -305,8 +305,8 @@ submissionHelper tid ssh csh shn mcid = do
     csheet@(Entity shid Sheet{..})  <- fetchSheet tid ssh csh shn
     maySubmit  <- (== Authorized) <$> evalAccessDB actionUrl True
     isLecturer <- (== Authorized) <$> evalAccessDB (CSheetR tid ssh csh shn SSubsR) True
-    case msmid of
-      Nothing     -> do
+    case (msmid, sheetGrouping) of
+      (Nothing, Arbitrary maxBuddies) -> do
         submissions <- E.select . E.from $ \(submission `E.InnerJoin` submissionUser) -> do
           E.on (submission E.^. SubmissionId E.==. submissionUser E.^. SubmissionUserSubmission)
           E.where_ $ submissionUser E.^. SubmissionUserUser   E.==. E.val uid
@@ -331,12 +331,23 @@ submissionHelper tid ssh csh shn mcid = do
                     E.&&. submissionUser E.^. SubmissionUserUser E.!=. E.val uid
               E.orderBy  [E.asc $ user E.^. UserEmail]
               return $ user E.^. UserId
-            return (csheet, Set.fromList $ map (Right . E.unValue) buddies, [], maySubmit, isLecturer, not isLecturer)
+            return ( csheet
+                   , buddies
+                     & map (Right . E.unValue)
+                     & Set.fromList
+                     & assertM' ((<= maxBuddies) . fromIntegral . Set.size . bool id (Set.insert $ Right uid) (not isLecturer))
+                     & fromMaybe Set.empty
+                   , []
+                   , maySubmit
+                   , isLecturer
+                   , not isLecturer
+                   )
           (E.Value smid:_) -> do
             cID <- encrypt smid
             addMessageI Info MsgSubmissionAlreadyExists
             redirect $ CSubmissionR tid ssh csh shn cID SubShowR
-      (Just smid) -> do
+      (Nothing, _) -> return (csheet, Set.empty, [], maySubmit, isLecturer, not isLecturer) -- TODO: Return registered group members
+      (Just smid, _) -> do
         void $ submissionMatchesSheet tid ssh csh shn (fromJust mcid)
 
         shid' <- submissionSheet <$> get404 smid