@mbeiswenger Suggest you take the free TryGit course. You can also consult the Git documentation and ask questions here (search first!).
In terms of collaborating, it depends if you will all be working on the same or different features. You will want at least one fork. Also, make your commits small and related (not “here is MM3.0” but “added optional config param capitalizeAll to newsfeed“). You can either share contributor access to a single branch, or all have them fork-your-fork and submit Pull Requests to your fork/branch. You shouldn’t need to modify .gitignore – it should already ignore the right files plus a lot of OS-specific cruft.
If you ever intend on submitting your changes to the MM develop (not master) branch, see the contributing notes in Contributing.md.