Certain Usergroup Upon Signup?

[CrazyTech]

I'm trying to setup a forum where when a user signs up, they can automatically be put in a certain usergroup with certain permissions based on an option that they chose. What's the best way to do this?

IE:
I have red group and blue group. Red group can only post in 2/10 forums while blue group can post in them all. I want blue and red to be set at signup so it automatically works.

I've been doing hand admin approval and placement because of some past attacks on the forum, but I would like to create an automatic system because we've grown so much lately.

Any help is truly appreciated!