You can start by talking about the idea on LFN on discord: https://discord.gg/pEzAuNV
and the music room in PAL https://discord.gg/Df4pDaC
There are other ways you can promote the idea, and that is by setting up your own discord and inviting musicians on steemit to join and collaborate.

Try to make the process as easy possible for people to join and start. People like to join moving wagons, so think about that when you present things to them. I still think they will be more attracted to the idea if there are prizes and stuff.

I'd like to help more but I'm busy next couple days, hopefully this would have gained steam when I come back here.