Upon first logging into the game, players will be able to pick any home world server available, including those in a different region from where they purchased the game. All characters thereafter are created on that home world. World transfers are possible, but have both a time and gem cost. A player can change home worlds only once every seven days and must shell out 1,800 gems to do so. Once the game goes live, however, a "guesting" feature will be available to allow players to join friends on any world, although players won't be able to participate in WvW for their temporary host server.
Guesting won't be in for the beta weekend event, but players will be able to test out transferring worlds. Each account will be able to unlock 2,000 free gems for the BWE, which will cover the cost of a transfer and a little more.
A complete server list for the upcoming beta weekend event is available in ArenaNet's blog post.