You do not have to Invite Company Admins to every Project within a TiM Company Account. A Company Admin will appear on a Project's Crew List once they click into the Project for the first time.
A Company may have dozens of Company Admins, and dozens of Projects, but not every Admin is managing every Project. To ensure only the Company Admins that are working on a Project are included on the Crew List (and prevent extraneous names from appearing on a Project's Crew List), TiM only adds a Company Admin to a Project's Crew List once the Company Admin has interacted with that Project.
Company Admins can access new Projects via the Company Account drop down menu at the top of the screen.