What you're about to do
Giving someone access to a project is two choices made together: the role (what they can do) and the scope (where they can do it). This guide walks through the Assign Roles step where you make both.
You reach this same step two ways — when you add a user (it's the last step), and when you edit an existing user's access from the User List. The steps are identical either way, so follow this guide from whichever you started.