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.
However, this idea eliminates the risk that your uers won't bug each of their friends to install your app.
Also, the right design for the email notification can act as great marketing for your app. You can certainly make a better landing page on your website than your app's product page in the App Store.
Besides, I am using Android now ;-) so...
Just a side note, for solutions that needs social networking to work, it would be extremely hard to charge people up front. You gonna have to give it away for free (first) to earn some viral effects to kick in.
If all have to buy a copy, the decision to buy would become a collective one which the chance of buying this app would be even lower.
Further, as this is the first app by the author, there is no track record as a reference.
Hope you understand my reasoning for why social apps should be free at least at the beginning.
This is why about.me demolished flavors.me, even though flavors.me came first.
Since your product is built around doing things with other people, there's a natural advertising angle, viz., suggesting things for people to do.
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.