How to let loggedin user join a site and choose a role

How can I allow MS users to join a site in the network while they are already loggedin. I have a site within the network that needs to assign a user role at registration but all users are required to register with buddypress on the main site. Are there any tools that will allow members to join a site.

I’ve seen there are little hacks that will automatically assign a role to a user if they land on the site but I need to give them the ability to “register” again to choose one of two roles that they need to be assigned to participate on the site.

Any help