Hope you're feeling better, cutie-pie!
Actually, I believe some user groups do exist, they're just transparent to users.
Usergroups, group users (ha!) by the way they use the system, and give them permissions according to the way they use the system. For example, there are guests, who have permission to look at posts. Then there's the member group, which can post items and edit their own postings. Then there are moderators, who can edit other people's posts, among other things. Then there are administrators, who have lots more system privileges.
We just don't have any additional usergroups, like ballroom, or salsa, or ballet, or competitive dancers. Yet.
I hope that helps clarify a little bit.
Jenn