I would say, explicitly enter zip code as a first approximation. Easy and not something people generally have an issue with (they can enter work or nearby). The site put me in California, which I am not. I think you could get better traction with more accuracy, and zipcode will do it.

That and an easy way to add your favorite venue near you (since it looks like there is only one venue in the US outside of CA).

Visually and concept wise it looks interesting. I bet there are outdoor adventure meetups in most cities that would be interested.