Users live under the User Management menu. A user record carries: the person's name and email — the email is the sign-in, and where notifications and password resets go (1.13); a password, which you set when you create the user and they later change from their own profile; their company; their home branch and department, which become the starting values on documents they raise (1.13); a link to their employee record, when they are one (Chapter 8); their roles (below); a preferred language; and two switches — Active, and Accountant (external).
You bring a person in by creating their user; there is no invitation queue to manage. Deactivate rather than delete when someone leaves: an inactive user cannot sign in, consumes no seat on your plan (15.2), and keeps their name on everything they ever posted — so the trail (16.5) stays truthful. The Accountant flag marks an external accountant: plans include dedicated accountant seats (15.2), so your bookkeeper's access does not eat a staff seat.
A role is a named bundle of permissions — the individual can-dos (view, create, approve, post, export…) of each module. Give a user one or more roles and their screens assemble themselves from the union of them: what they may not do is simply not there (1.12).