assigning roles with memberships

Here's what I'm hoping to do...

People can join my membership site for free to see the protected content. But, free members will still have to pay with a like to see this content.

Paying members won't have to pay with a like to see the protected content.

I thought I could do this using roles (i.e. "authorized users see full content" in Pay with a Like plugin settings), but I don't know how to assign this role level ONLY to paying members when they join.

Maybe there is an easier way?