Everyone who has joined a Sponsored Group has opted in by default to receiving Facebook "email" messages:
Here's the latest Valentine email from aerie, a clothing line from American Eagle aimed at 15-25 year old females. It was sent to almost 50,000 members of its Sponsored Group:
One problem with Facebook groups is it's hard to keep members engaged. Most people join and never return. This email gives members incentive to return to the page and enter a contest, see what's new and start playing around with the "Kissing Booth" application (link removed, application no longer active).
Kissing Booth is a SuperPoke clone where you can send virtual kisses to friends and you move up in ranks the more kisses you send and receive. For example, you begin as a "cutie" and work your way up to "flirt." The genius of this is that when you "kiss" someone, they must add the app to kiss you back, spreading the awareness of the brand's Sponsored Page, which in turn builds the Facebook "mailing list":
The aerie Sponsored Group (link removed, group inactive) is well designed, appropriate for its target market and has a number of ways members can interact. There is even a link to a new blog for employees, which could appeal to future sales associates.