Open Administration, Organization, then Users to create an account and give it its roles, on one entity or on all of them.
The number of users is capped by your plan, plus any extra seats bought. The cap is checked on every creation path, invitations included, and it also counts people from another company you have given access to.
A plan with no declared cap imposes none. If the cap is reached, the Subscription screen shows the plans and the seats you can add.
The Roles screen defines what each role allows; the Users screen gives roles to a person. What they see depends on those roles.
A role whose Mandatory two-factor authentication box is ticked — Administrator has it by default in a new company — requires the second factor. It is set up at the person's next login.
PlansIncluded in plans: Groupe, Entreprise.