unfortunately at the moment you can only import contacts from GMail/Yahoo,
but I'll add this request to the backlog so we can get to it ASAP.
however, once an address exists in the system, you can simply search for a part of it and go to that user's page and click "invite to contact", which is a bit easier.
a bigger overhaul for the invitations UI is planned, but might take a while to do.
cheers,
Yonatan