I wouldn't recommend this hack. I posted a reason in another thread but I'll repost it here in case anyone misses it.
Deleting groups isn't a good idea, which is why deleting a group was never an admin function. The U_Groups field is a varchar field, max limit of 255. The G_Id in the Groups table is an auto_incremented field. To make it so a user can belong to the maximum number of groups .threads enforces the max group # to 99, 2 digits in length. When you delete a group that G_Id will not be reused since it's an auto_inc field so eventually you'll end up not being able to create any new groups. That's why in 6.4 they don't actually delete they just get disabled.
