BuddyPress.org

I don’t exactly understand your idea here.

You are free to start any group you want. Then you can ask your friends to join that group if you’d like.

Are you saying that Xing can ask non-registered email addresses to register, and when they do they are automatically added to the group you create?

Say you create a group called “We love mustard” and I have friends outside of BuddyPress that also love Mustard, that I could request their presence in the group via an email address, and when they register they are auto-joined in the “We love mustard” group?