This server based plugin will allow you to make your Bukkit multiplayer server a "Invite Only Environment". Basically, how it works, is you enable whitlisting in server.properties. Add the permission node/'s below to the specific group you wish.
/invite PLAYERNAME /invitedev
None at the moment! Wahoo!
If you would like to see more features, email me at [email protected]. Again if there is a lack of development is because I am busy, or working on more projects for my fellow Bukkiter's