In the Ad Accounts section of Business Settings, you’ll see ad accounts that you own and ad accounts that you have access to.

There are 3 ways to add ad accounts to your Business Manager:

1. Add an Ad Account: Adding an ad account moves it permanently into Business Manager. You must be both the owner of the ad account and an admin in Business Manager to add the ad account. Please note, once you add an ad account and it's moved into your Business Manager, this action can't be reversed. All management of your ad account must be completed within your Business Manager profile. You can't add an ad account that's owned by another Business Manager. If you still want to work on an ad account that's owned by a different business, you can request access to it.
2. Request Access to an Ad Account: If you request access to an ad account in Business Manager, the admin of that Business Manager can grant you permission to work on it.
3. Create a new ad account: If you create a new ad account in Business Manager, it'll permanently belong to that Business Manager. Once created within Business Manager, the ad account can't be transferred to an individual owner who doesn't own a Business Manager.