One of the features Arenanet has promised its guild wars 2 players since the start was the guesting feature. This feature allows a player to participate in the PVE of a different server while having his/her character data saved onto his/her original server which was chosen at the time of character creation. This allows the player to play with a friend that had picked a different server from the start.
Normally you would have to permanently server transfer yourself to play with such a friend which Guild Wars 2 currently gives you the option to do so for free but at a 24 hour cooldown.
Guesting does not allow a player to participate in another server’s World versus World map; this is to stop potential griefing and promote server loyalty in the battleground. The only option to play on a different side for WvW would be to partake in a server transfer.
This feature is said to come at the 28th of January and with it will be the removal of the free server transfer option mentioned before. It will most likely cost money so it would be wise to be sure in which server you want be on. Remember, server transferring will result in starting your guild from scratch since guild upgrades and influence are server based!