Registration process: Allow user to select user group?

[razec]

I'm looking to modify the registration process slightly but I'm not sure where to begin with it. When users sign up for the board I'm designing, they need to be able to select the type of user they are. Based on this selection, they're assigned to one of several corresponding user groups or redirected to the subscriptions page (one of the options is a recurring paid monthly subscription). I'm thinking this would need to be a second step to the registration process (presented to them after they verify their account).

How could I add this so it happens automatically, like after they click the confirmation URL for their account?