Nevermind. I am dumb!
Actually, you can add roles to groups. Just go to your group under People and Groups. Choose New -> Add Users. And then just type in the name of the Role you'd like to add to the group and (as long as everything's setup properly) it should be recognized.
If you're only going to be using the FBA users within SharePoint, you might want to just add the users directly to the SharePoint groups to cut out an extra layer of administration.