Assigning user to one project only

[Fergus]

I have many projects - all private

I want to assign a user to items in one project only and have the ability to create new items and update and close them, but not enough ability to mess up any settings [this user is not tech savvy. but not change any items assigned to someone else.

I have attached the user to the project as a developer - but I can't add that user to a new project item.

Also if I assign myself to the project item, I can then go back and assign "blank user" and it shows the item as assigned with no name