As many said, just simply shout for a group vaguely and you'll get an invite. Theres a few things people do in there but they're all more or less the same... farming fates.

Castrum will be a problem if you try and find a group in the zone. Create or Join one on your server's Party finder. Typically people will have an alliance fill and then join instances until they find an active or soon to be Castrum to join. I know you're new and probably don't get this yet, but you don't want to join a Castrum under 40 people. It's a waste of time and difficult enough if it's full and most people are new to it.