You can now apply to become a beta tester for the TaleWorlds' Mount & Blade: Warband (the upcoming expansion pack with multiplayer features) by going to the sign-up page and filling up a short application form. Preferences will be given to those who provide valid serial keys, and also to applicants whose location are close to the multiplayer servers' locations.

M&B Warband Beta Testing Invitation
Mount & Blade: Warband GDC presentation