I'm working on a new site and would like to keep a subset of members from doing anything except in one or two modules. These members will have email-cofirmed registration. I've modifed the login process to allow these members to select if they want to be in this subset. Once their registration is confirmed I'd like them to have access to just a handful of modules. I'd like them to be able to access module a,b or c but want to log them out if they access any other module. Is the easiest way to gain this functionality to put them in a special group? If so can I do this at registration (after they select Finish but before first login)? I've thought I could put then in another group but was wary of changing the registration mechanism..but could I break anything by putting them in another gid before they even login?

If you make the default group only have access to less areas, you could manually move people you want to have top access to another group.

This is what I do currently and move people up after a certain number of posts etc.

Automating this would be a real time saver though, how feasible is this?

I guess that you can use the Subscription module(without subscription fee function). I have no experience using it but it promises what you want. Subscription Module
Let us know your results!

Thank you, this may serve several purposes as I was thinking of adding some value added features/modules to stimulate small subscriptions as well.