For your question "Otherwise, I see no option to activate a company to assign users. "
In the User management form, the administrator can select several companies, to which the user has access, in the « internal companies » panel
Then the administrator can assign a default company to the user.
WARNING : Make sure that the user’s role has perm.user.Auth.rwc, perm.base.Company.r rights, so that the user can possibly change the company, in their preferences.
Otherwise, in the logs you have a message like « Authorization error: you are not authorized to read this resource: com.axelor.auth.db.User »
If the user has the permissions, he himself can change the active company in his preferences, among all the companies authorized by the administrator.