Hey Peter, thanks for submitting this! Users can click a "Join" button on the activity from within the app, which will add them to a list of activity attendees. That list is viewable on the activity (only to you, as the host) by tapping the "spots" link to the right of the town name.

If you have a separate place you'd like them to ultimately register, such as Sign Up Genius, you can include a link in the activity description.

I like the idea of displaying "interest." Perhaps this could be a number of views of the activity, even if the user didn't join?