Another angle is a groupon type service. Do something fun together, get a coupon for food/drink/another fun activity that unlocks when all members on the group

Another angle is a groupon type service. Do something fun together, get a coupon for food/drink/another fun activity that unlocks when all members on the group reunite in a month. Create mechanism for 'chaining' to encourage stickiness. People get slight push to maintain friendships they might let otherwise let slip away.