In my experience, this is usually a bad idea, as it has the tendency to limit who will/can be a part of the group.
"But," you say, "that's exactly what I'm asking for!"
Here, I would ask if that's really what you want. Do you intentionally want to turn potential new members away? Is that the best thing for the community?
I have a strong dislike for programmatic content filters. If we must have an editorial policy on too-similar usernames (like, for instance, tye or im2.), let's make it part of the consideration system. This is not the sort of thing that belongs in code. I'm not convinced that it's the sort of thing that belongs anywhere else, either.