Hi Ray,
Thank you for reaching out. The other accounts could either be linked to your MCC account or they can just create their own OAuth2 credentials and issue the API calls themselves. If their accounts are linked to your MCC account and you have the admin or standard access to their account then you could make calls on behalf of them. Otherwise they need to create the credentials and call the API themselves.
Thanks and regards,
Xiaoming, Google Ads API Team