Please see  https://digitalshift.helpninjadocs.com/article/give-partner-access-in-your-facebook-business-manager-account for the recommended method.

 

Alternative method (not recommended): 

Add Admin to Your Business Manager

To give people access to your business:

  1. Go to https://business.facebook.com/settings/.
  2. Click People.
  3. Click + Add.
  4. Enter the work email address of the person you want to add.
  5. Select Admin access.
  6. Click Next.
  7. 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. In the third column, toggle on each task you'd like to grant them. Or, toggle on admin access to allow them to manage the asset and perform all available tasks. You can add people to multiple assets on this screen by repeating this step.
  8. Click Invite.