In a foursquare Friends special, you set the minimum threshold for a group of foursquare friends you’d like to reward for visiting your business together, and the Special is only redeemed if that many friends check in (based on a user’s foursquare friends)

This is a great way to get groups of foursquare friends to come to your venue, and to working off the social aspect of foursquare to drive people to your venue.

Check out this screenshot for a great example -