Thanks for raising this question with us around how to invite users to a project. Since projects reside within and inherit the general permissions of the parent place, then members of that place will also have access to the project. There is no invitation functionality within projects. You may share the project if the intent is to simply notify users of its existence or remind them, but otherwise members of the place will already have access. Users who do not have access to the place will need to be invited to the place in order to access its projects.
I hope this makes sense? Let me know if you had any other questions.