See if you can work with data from Strava. Generate groups of people that do similar activities in similar areas, and then somehow invite them to group rides. Those that show up could be a great start on getting some network effect.

Though to be honest I have no idea what kind of info is available from Strava, but I feel that the more you can do on your end and the less involvement required by users, the further you could get with a greater number of people. Relying on people coming to you to sign up for yet another social network seems like a lot to ask of people.

This idea seems like it's 100% about distribution. If I owned an extreme sports rental shop, I would have an incentive to get more people to go out and participate in the activities. If the app was well made, reputable and secure I would consider putting up a sign in my store or whatnot.

People that run equipment rental stores probably have a facebook group or professional association. If you can befriend someone influential in one of those, it might be a good place to get started.