Perhaps you want to take an exclusive lock on the table? The operation you describe seems to suggest that you'd want to guarantee exclusive write access to the table.
Exclusive table lock is a bit excessive IMO. Locking particular group should be good, though it is not quite straightforward to achieve. I'd use advisory locks or would lock a row in a parent group table (if such table exists, if not - it might be worth to make one) referenced by rows in question.
