Awesome idea. What about an app for organizing small, informal get-togethers. For example, I host regular board game nights. It would be nice to have an app that organizes the event and sends notifications to guests and me (the organizer). The organizer could suggest a list of possible dates that the guests could pick from. If it's a potluck-type event, the organizer could specify a list of food/necessities that guests could volunteer to bring. It could then send a reminder the day before to confirm. Bonus points for calendar integration (maybe ical format) and sms notifications.
Meetup is great, but I feel like it is targeted at more formal groups.