If you're talking about Planetside Universe, there's no 'main' server that I know of. Grumpy has thrown
this thread up so you can always feel free to hit him up.
If you're just looking for an outfit to join, my suggestion would be to hop onto the nearest server (Connery is West Coast US, I think Mattherson is East Coast US, Miller and Cobalt are EU and Briggs is home to the homies down under, for example) and start joining random squads until you find a group of people you enjoy playing with.