Select “Clients” and then “View Clients” from the menu at the top of your screen.
Select “View” beside the company you would like to add the multi-user to.
At the bottom of the page you should see the section called “Account Multi-Users”. Select the “+” button on the right corner of this section.
Now you will be brought to the “Existing Users” page. You can now search for the existing user that you wish to add to this client.
Once you have found the user you are trying to add you are able to select the “Add User” button beside the user.