Jane, I don't think it's a lot like tagging photos. Sure, you can get tagged in an embarassing photo, but as long as it does not belong to a private setting, you can't really do anything to prohibit the exhibition of the photo, even if you untag yourself. But there is an undeniable fact here: you really are in that photo!
Now, a group can be about anything, especially of things that you want absolutely nothing to do with, and I forsee people having to log on to facebook more often to avoid getting in trouble with their employers, for example - suppose a disgruntled ex-employee sets up a group against the company you still work for and adds you to it - see the problems coming ? And this is just an example. This can go to politics, religion, etc...
This system will be changed, rest assured, or many people will just get out of Facebook. And Mark Zuckerberg won't have that. lol