If you are a Manager of your company, and a  Manager of another company gives you the profile of Manager to his/her company, you can administer it.

You can thus combine the administration of several companies under the same username.

Note: even if you are a Manager for several companies, you can create establishment groups only for establishments within the same company. You cannot create company groups.

If you are an Administrator within a Head Contractor, you can administer all the companies of that Head Contractor, but them alone.