BuddyPress.org

I currently have 4 sites with individual buddypress and paid membership pro allowing users to sign up. We would like to be able to offer each site the ability to pay a little extra and be able to either be a member of each one of the others or all of them using there same profile name that they currently have. what would be the best way to do this. I have read about using multi site but i would like others that have had more experience to give me there input on what they think would work out best in our case. we still want each site to have the ability to have users just sign up for that site or select all sites at there choice. Any input would be greatly appreciated.