A number of people already participating in the beta have received an additional beta key which allows them to invite a friend.
Short quote of the e-mail:
"Congratulations! This is an additional invitation to the beta test for Blizzard Entertainment's StarCraft® II: Wings of Liberty for you to share with a friend! You are receiving this email because you are currently participating in the StarCraft II beta test, and we want to encourage you to test out some of the social features of the revamped Battle.net® online-gaming service by inviting one of your friends to play with you."