What we've been doing since the game came out last year, was play it multiplayer. We've never played the game SP.
Currently we've got two private servers running concurrently the Eye of the Beholder v1.1 module. We haven't yet ventured onto any public servers as we haven't had a big enough crew to require it.
One server is full, a second has a spot open. I don't know how invitations work.
MP is definitely more fun than SP.
We play almost like pen and paper AD&D. Voice comms are supplied through Roger Wilco, and one of us servers that. The only difference is that you actually see what you imagined what was happening.
PlanetSide 2 | Briggs (AU) | Deromg