We need a tool that will allow inviting all contacts that user of the tool have in services like facebook, gmail, twitter, plaxo, etc etc
It should be simple for users to understand, not more than 2, eventually 3 steps to finish the invite and sharing process.
If it can allow management of the invites, even better. So that user can quickly see who accepted the invitation.
There are tools to do this already, your job will be to integrate and customize whatever necessary to make sure it will work in a very simple way.
Will require a very very detailed information about the tools you will use and a mock-up of how it will look like for users, or, even better, a working demo.
Will ignore "can do its" or answers that do not respect request above.