It's best if you find groups with the same interest, such as end-game raiders, crafting, or even fishing specific. You can be in one Free Company (guild) at a time, and you can be part of up to 8 linkshells (private chat channels) at once also.

Like Mithras said, just /shout what you're looking for, or you can go to the Brynhildr world forums. The world forums often have many recruitment threads. Or just start singing the lyrics to a song in /shout, it usually catches on pretty fast XD