Once you have confirmed you're "down," you can add a status message to say just what you're "down" for, be it a quiet evening with a few close friends or a booze-fueled night of debauchery that doesn't end until someone's in the hospital. Naturally, there are group chatting features included, as well.
The problem it's trying to find seems very similar to what Apple's Find My Friends app and Foursquare's Swarm app focus on, though neither of those explicitly let you say you're "down" for a specific activity. Unfortunately, Who's Down is currently invite-only, and the app's page on Google Play doesn't have a whole lot of detail on how exactly things work beyond the included screenshots. We're not quite sure how it matches you with your friends, for example -- in all likelihood it works with your Google contacts or people you chat with on Hangouts, but until we can give the app a spin ourselves, we'll have to wait and see. If anyone's "down" to send us an invite, feel free!