how to determine which group?

I am setting up 4 user groups - Admin, Moderators, Guest, Users.

I want Guests to have read only access from the point they register. I know that I can administer an individual member and put them in a certain group AFTER they register, but is there a way to put specific users into specific groups at the point they register.  The forum is a subsite of an intranet application. When users enter the application I already know who is read-only.  So I think I need a way to use that data to automatically set someone into my Guests group at the point they register.  I'm suspect there must be a file I can go into and do this in some sort of conditional, but I don't know which file.  Can anyone help me out here or suggest another way to make this happen?