If your colleagues already have access to the platform, however, they need to be given access to additional accounts, you can do so by following these steps (screenshots below)
:
Click on the Admin Tools tab in the left-hand navigation bar
Select an Advertiser they can already access
Click on the Users tab on the left-hand side
In front of their name, on the right side of the table, click on the three dotted lines, in order to edit their access
Under User Access on the right side of your screen, click on the edit pencil
Tick Ungrouped, then tick all the accounts you would like them to access. Make sure you click on Save, then Save & close
Check out the screenshots below to help you!