Maybe create a few “first issue” issues. Might encourage people to start contributing if there is something concrete to start with. Otherwisr, could be a bit overhvelming just clone the repo and start contributing.

In Grayjay you can group your subscriptions. https://grayjay.app/

Yes it is. But people can’t share files unless you allow it, or you create a user for them. I think everybody can share files using Send.