02 Jan 2006 at 17:28 - 2
1. You join a guild either by being invited to a guild by an officer; or by creating your own guild at the Guild Registrar (who is in Ascalon City, Lion's Arch, big cities). If someone sends you an invite, hit G to bring up guild screen where you can accept.
2. There are arenas where players fight one another, but I am not sure where you heard about gambling items. All of the arenas can be accessed by PvE players (once they physically reach the location in-game). PvP-only characters can access only the level 20 arenas.
You gain experience and faction by killing and winning matches. There are random arenas, where you are put on a team automatically, and there are team arenas where you form a group before you enter. These are always 4 v 4. There are also 8 v 8 PvP options such as guild battles and the Tomb of Primeval Kings.