I built this in 37 days with no prior iOS experience (though I did have experience with ruby/rails). I think it could be useful but I'd love some feedback.
I would consider some sort of free version of this app, so people can join without risk. Maybe make a version that limits you to 3 friends for free, or unlimited friends for $1.99?
I suggest a fremium pricing model. Perhaps all features are free exept one killer feature, which is an optional in-app purchase. For example - push notifications when one of your friends suggests an activity.