There are around 30 or so people contributing ideas and what have you to the project, or wherever we can. Those listed on the actual site are just people who committed code to the repo. A lot of us play BotW though :p.
Daeken posted on an HN post regarding the Switch before launch, that's how I found it and joined. A lot of members have joined since then, mostly by invitations from people within the general 3ds communities I suppose.
Actually, this one grew totally organically. I used to do console hacking many years ago, and decided I wanted to get back into it with the Switch. People just started joining up, and bringing in trusted folks with them. A comment I made on HN a month ago or so really accelerated this.