I looked this up (multiplayer portal) about a week ago.
Currently, Valve hasn't released any SDK for Portal, which makes it difficult to make any mods for Portal or based on Portal.
If you look on Youtube, there are some videos of people trying to do it anyway. But I couldn't find anything showing people using portals with other people. Just people running around with the Portal player model and being able to pick up barrels and other crap. Nothing major.