Hello,
Thank you for reaching out. With regards to your concern, a manager account can access invitations to other client accounts as mentioned in this article and use the developer token associated with the MCC account to make API calls. The MCC account can grant the access levels to their managed client accounts as listed here. You could also use API to extend invitations to the client account, please refer to this guide for more details. Let us know if you need additional information.
Regards,
Nikisha Patel, Google Ads API Team

Hello Sonya,
Please refer to this guide for extending invitations from the manager account to the client accounts via API. You might find this code sample useful to accept the invitation link from the manager account to the client account. You could use ManagedCustomerService.mutateLink to link the manager account and client accounts via API. Let us know if you have any additional concerns.
Regards,
Nikisha Patel, Google Ads API Team