Billing contacts can only be added by team owners. However, team admin can escalate his privileges and add billing contacts.
Steps to reproduce: 1.Log in as team admin 2.Send the below request using his token and it adds 'email@example.com' to billing contacts.
POST /api/team.billing.addContact HTTP/1.1 Host: satishb3mailinator.slack.com Content-Type: application/x-www-form-urlencoded; charset=UTF-8 Content-Length: 106
To confirm, login as team owner and navigate to billing contacts. Notice that firstname.lastname@example.org is added to billing contact list.