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Ok, if anyone else gets this, it's probably because you did what I did and deleted a user row from the database without realizing that that user is also linked to group_user_link... which is where groups.php gets its listing of users in the group.
So, it's a data integrity issue.
I recommend updating groups.php to check:
if (the uid of this groups_users_link is invalid)
then (do something other than throw an error; either put a fully-descriptive message like "UserID x has been deleted from user table; please delete records from groups_users_link table where uid=x" or simply delete the extra entries automatically)
In any case, glad to have it solved.
Good day,
Keith