Where would you look to find communities of musicians?

I'm currently working on an app which would help musicians collaborate schedule with each other easier then via email/phone etc.. Also for groups like Bands, Choirs etc.. But I'm having a hard time finding the right research about how many serious musicians are out there doing it for a living, Where would you look as a start? I've tried searching for "band forums" "musician forums" etc.. But that doesn't help much