You can add a user to multiple companies with the same email address. Because a user can be invited only using one email address to access multiple companies.
But please know that a user can only have a single user ID since a user ID is equal to a single email address.
To do so:
Here's an article you can read to learn more about how you can add and manage a user's access: Add, delete, or change user access.
Keep me posted in the comment section down below if you have any other questions. I'm always around happy to lend a helping hand.