Add a partner to Facebook Business Manager
When you manage more than one Facebook Page (or Ad Account), you probably use Facebook Business Manager. It's the right tool for the job: it allows multiple people to work on concepts, ads and manage campaigns. But, in time, you're going to want to focus more on your business, and get somebody else on board, like an agency. But then you think, think, “I've looked after this page well, I really going to hand it over to an agent now?” So, don't let the agency own your page, rather add them as a partner to your Facebook Business Manager account.
“Will I keep ownership and admin rights?”
Good question!! Yes, the whole purpose of this is to show you how to add a partner to your page, without giving away control of your page to somebody else.
Steps to add a partner (with pictures).
You can complete these steps separately if you need to ad a partner for your Pages, Ad Accounts, Pixels, Product feeds, Apps and your Instagram Account.
Step 1
Login to Facebook. Go to your Business Manager. Click “Business Settings” on the top right.

Step 2
Under “Accounts” on the menu, click on “Pages“ (do you see that you can also do this for ad accounts, apps, Instagram, projects etc.)

Step 3
Select the page you own and find “Assign Partner” to the right.

Step 4
Select the role you want to assign. This will generate a link. Copy the link and send it to your Facebook Business Manager partner. Click close when done.

What role should you assign?
That depends on what you want the person to do. When it comes to advertising, the following are relevant:
Admin
View ads, access reports, create / edit ads, edit payment methods and manage admin permissions.
Advertiser
View ads, access reports, create / edit ads.
Analyst
View ads and access reports (just view, can't make changes)
What about my Pixels, Ad Account, Apps, product feeds, Instagram?
You can add a partner to your pixel, ad account, app, product feeds and Instagram account by following the steps above.