You can only become a general user over someone else’s Facebook ad account. To get that access you need to contact the admin (the owner) of that Ad account and ask for access. The admin of the Facebook ad account will be able to give you access in the Settings tab in the Ads Manger on Facebook, under “permissions”. Learn more here.
If you are not friend with that person on Facebook, be sure you provide with the email address you use on Facebook.