So you're looking to get your agency on board and working with your Facebook business Advertising manager. Here are the steps to follow:
Step 1 Business Account Access:
Follow this link: https://business.facebook.com/settings/people/
Click on your brand you would like to give access to.
You will see a users drop down on the left:
Click on 'Partners' and then 'Add'
Select 'Give partner access'
Add your agencies ID number and hit next
Now you need to work down the assets you are giving control over. Start at 'pages' and work down the column to 'new pages experience', then 'classic pages', then 'ad accounts' and so on until you get to to the end.
When a page is available to select under the 'Name' heading, select it by clicking the dot next to it:
Then turn on 'Full control' at the bottom of the far right:
If two accounts appear under the Name heading, select both and turn on full control.
At the end you should have a few accounts selected. Then hit 'save changes'