Anyone that has participated in a online match during the current run of the Gears of War 3 Beta will receive a code via email, which they can offer to a friend to allow them access to the Beta.
The 'Buddy Token' will be released to Beta players on May 8, and will signal the charge towards the Beta's closure come May 15. Microsoft point out that some weapon skins and character variants will only be available to unlock in the Beta, though they'll roll into the game proper when its released September 20.