Users are invited to DTM through an Admin user. When a user is invited to DTM, an invitation link is sent to the email address specified during creation. To activate the invitation, the user has to click the link. If they do not click the link the user account shows as 'invited', but not as registered.
For more information on inviting users check the documentation here: https://marketing.adobe.com/resources/help/en_US/dtm/t_users_permissions.html
After the invitation has been accepted, the user appears as a Registered User.
A single user can span multiple companies and properties.
If a DTM account has not been created for your company, follow the steps here.