Truly I don't understand how a user group works vs. a forum topic area?
But I like the whole international melting pot we have going here. If I were to try to start a Michigan group hang, I can just post it in the hangout get together section and I would welcome anyone who wants to come to the hang... not just michiganders.
The GoKart example seems like the Roundtable Off topic would be the perfect place for that. That would seem to be a pretty small cross section of the community so a user group so specific seems like overkill. Thats just my opinion, not trying to be rude. If I want to find something specific like Gokart hammock camping I go to google and search hammock forums ie: site:hammockforums.net gokart , and if anyone ever mentioned gokarts it will come up. And not breaking into user groups but just having it come up in the "new posts" or "todays posts" means I might see something I didn't know I was interested in (gokarts) but now suddenly am, so I click in to read more.
The gokart example was just off the top of my head, and probably a bad one. Yes, the area hanger's groups are probably the best example. Those should be public, IMHO. If off topic user groups where allowed they would be private for donating members only.
"Good people do not need laws to tell them to act responsibly, while bad people will find a way around the laws." -Plato