So, I saw the "Implementing permissions across multiple user groups" thread by Brogan. It looks like a great idea, and it's motivated me to clean up my own forum's usergroups and permissions, but...doing so will require that I change the usergroups for (low) hundreds of users. In the end, I believe it will save me time even if I have to do it one user at a time, but I'm hoping there is an easier way.
Mass user management isn't yet possible, it is something that is planned though.
Unfortunately for now you will have to do them one at a time.
Bear in mind though that if you delete a user group which is a primary group, all of the members will revert to the default registered user group as their primary.