Holler falls somewhere between group messaging apps, event planning tools and “social discovery” sites. The main action is for a user to broadcast — “holler,” as it were — that they are down to hang out, either to specific friends or to people nearby who have said they are interested in a certain activity or event (dog walking, surfing, a conference).

Then the recipients get push notifications and, if they’re also down to hang out, join an unstructured chat to establish when, where and how they’ll find each other.

O’Neill, who is funding the four-person, San Francisco-based Holler himself, said in an interview today that he thought he knew what he was getting into but really, start-ups are way harder than they look. After some setbacks, he taught himself how to make an iOS app, which was just released into the app store this afternoon. There’s also a Web and mobile Web version. Check the screenshots below: