I assume that the groups suggestion on the master list is for group topics that people can join. I would like to be able to sort friends into groups (i.e. a group for other moderators and a group for FUNi staff etc.)
Due to my habit of never turning down a friend request my list is going to get quite cluttered eventually.
Being able to tag or group users in general (even if they aren't your friend, but not in a way that is visible to other users) might be useful actually.