I added a contributing.md to a recent open source group project for our local (Melbourne) freeCodeCamp group.
I recommend you add one of those so that people know how to contribute. (Steal mine and change the links if you like 
)
Since this is a greenfield project (i.e. you’re making it from scratch together), you should at a minimum put together a To-Do list of jobs that need to be done, so people have a sense of what you’re making.
Even better, create extensive User Stories, and use the GitHub projects Kanban style board to set out the road map of work to be completed.
New projects are exciting, but you’ll benefit from a good structure at the start to steer you in the right direction.