While it is technically possible to have multiple companies use a single account, we generally don't recommend it for a number of reasons. Some of these include:
potential for contact list cross-contamination, accidental sendouts, and mass unsubs/spam reports
having to change the legally-required physical address regularly between emails
having to change the From and Reply-To addresses regularly
My recommendation would be to potentially look at the Partner program, to maintain a master account with the various companies as separate accounts beneath it. This will allow those other companies to maintain their own lists, campaigns, library files, legally-required physical addresses, verified email addresses and users, etc. Having a free master account would also work toward your benefit by allowing you to create templates within it, then distribute them as needed into the sub-accounts.