Once you clicked on settings, you will see "Ad account roles" tab on the right side of that page.
Click on "Add People" button.
Once you clicked on "Add People", you will be prompted to "Add People To This Ad Account" pop up window.
Enter the Email ID of the account you want to give access to and click on "Confirm" button.
Once you confirm the request, that particular user will receive a notification for the same.
Facebook Ad Account Access Steps