ArenaNet have just announced that the highly anticipated "guesting" feature will be available to all players following the scheduled update on January 28th. Guesting will give players the opportunity to party with friends on other servers in the same region. January 28th will also see the end of free transfers to other servers; so if you're looking to transfer now is the time to do it.
Although Guesting offers a lot of freedom in regards to players joining friends, it does come with a few restrictions.
Because (different from a world transfer) we are not moving your character between data centers, you may only guest on worlds in the same region as your home world. Players on North American home worlds cannot guest on European worlds or vice versa.
You may not participate in World vs. World on your guest world. If you choose to enter World vs. World on the world you are guesting on, you’ll be moved back to your home world. If you want to play World vs. World on the world you’re guesting on, you must perform a full world transfer to that world.
You’re eligible to guest on only 2 different worlds at a time. Each time you guest on a world, that world becomes one of your eligible guest worlds for the next 24 hours. After 24 hours, you may choose to guest on a different world. You may log in to any world you are eligible to guest on as many times as you wish with no restriction.
Guesting on worlds that are full or highly populated increases your risk of being put into an overflow map.
For more information on Guesting in Guild Wars 2, you can check the official blog post here.