Co-founder of CoachUp, a two-sided marketplace between sports coaches and athletes, here.

We started in a single city (Boston), and just the coach side. The non-technical founder was himself a coach and had lots of friends who were coaches, so it was easy to get the first ~50 coaches across a handful of sports. From there we focused heavily on getting coaches to sign up with the pitch that it was free to them, gives them a nice profile / landing page they could put on their website or business cards or tell people about, and would eventually even start driving leads.

I think the cost/benefit was there for the coaches: little cost (short application) and some minor benefit even without the athletes.

this is exactly what I was hoping to hear, someone who actually did it. Starting with one side where the cost/benefit was obvious and letting the other side follow makes a lot of sense. How long before the athlete side started coming in organically?