-Login to your Facebook Business Manager account
-Go to Business Settings
-Click + Add.
-Enter the work email addresses of the people you want to add and select the role you'd like to assign them. Then click Next.
-Choose a type of asset in the first column, then select the individual assets you want to add this person to in the second column and assign a role for your Partner in the third column. On this screen, you can add people to multiple assets.
Here is a link with information about the different Roles for people in Business Manager.