Users, who need access to multiple company accounts, can be set up to use the same email address when logging in.
In order to set this up, you will need to first add the user onto the account as a brand new employee. Go to Company > Employees and click the blue plus button to do this.
Here, add in the user's first and last names and the email address they wish to use.
(Please note: If you just add the employee's name and then try to add the email in their profile later on, the system will not let you. You must add in the email address at this exact point for this to be successful).
Once added, the user will be able to login using their registered email. They will be to toggle between the different company accounts via the blue bar.
You can repeat this process when adding a user onto a different company account. As long as you add in the same email at the time of addition, it will sync the same login/email across accounts.