1. I don't want anyone I accept to join the project to be able to take my idea ... and go on their own
2. Is there some pre-made NDA's for this kind of stuff?
3. Does there need to be anything more formal than just a document?
4. Can they "digitally" sign this,
5. ie just type their name on it?
1. That's unlikely, but an NDA is reasonable.
2. Yes. https://www.google.com/search?q=NDA+template But it would be best to have an attorney's help rather than use one you just found online, especially if you've never signed one before.
3. Nobody needs to put any blood or shrunken chicken heads on the document, if that's what you mean.
5. That's not a good way. https://www.google.com/search?q=ways+to+sign+contracts