Help with ACL and CB user registration needed

I'm making a website with CB where I'll be giving free and paid subscriptions to my users. I've created user groups in Joomla ACL like "Premium Subscriber", "Free Subscriber", etc. which I will assign to my users according to their membership. What I want is that when a user registers from front-end, he/she should automatically have the "Free Subscriber" user group assigned along with the default "Registered" user group. Now how to do that?

You can only assign 1 registration usergroup. So to do this you'll need to use CB Auto Actions usergroup action on the after registration trigger to give them the free subscriber usergroup and they should then have both free subscriber and registered.