From regular Facebook, you go to your Ads Manager —> All Tools (Hover over menu left to logo) —> Business Manager:
Once you’re inside Business Manager, click on the “Business Settings” settings wheel:
Then click on the Ad Accounts tab to the left:
While inside Ad Accounts, look for the “People” section and click on “Assign Partner”:
Select the person’s role and then you’ll get the link to share with that person so they can be added to your account by clicking it:
They must have a Business Manager account or create one the moment they click the link in order for this to work. That’s it!