Hello @LaurenF243 ,
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:
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.
For more info on the Partner program, feel free to peruse this page and select a type most in line with how your organization / business structure would work. You can apply for the program through this form, which will allow a Partner onboarding specialist to reach out and provide some more specific insight regarding your setup and needs.