UCWA MyOnlineMeeting attendees requirements? External attendees

I'm currently trying to develop an application that creates Skype meetings.
I'm leveraging the C# UCWA SDK and developing against Skype for Business online.
Meeting creation works fine if I only include people from the tenant in attendees, as soon as I include people not from the tenant in the meeting I get this error message: {"code":"BadRequest","subcode":"ParameterValidationFailure","message":"Please check what you entered and try again.","debugInfo":{"diagnosticsCode":"2"}}

Here is my code sample

var meeting = new MyOnlineMeeting()
            {
                AccessLevel = AccessLevel.Everyone,
                Attendees = new string[] { $"sip:{Settings.SkypeUserEmail}" }, //Adding anybody else than the service account makes it fail for now
                Subject = series.Subject,
                ExpirationTime = DateTime.Now.AddDays(3),
                AutomaticLeaderAssignment = AutomaticLeaderAssignment.SameEnterprise,
                Leaders = series.Organizers.Select(x => $"sip:{x.EmailAddress}").ToArray(),
                LobbyBypassForPhoneUsers = LobbyBypassForPhoneUsers.Enabled,
                PhoneUserAdmission = PhoneUserAdmission.Disabled
            };
            var dialIn = await client.OnlineMeetings.GetPhoneDialInInformation();
            var meetings = await client.OnlineMeetings.GetMyOnlineMeetings();
            var result = await meetings.Create(meeting);

Adding external users to the organizers properties works fine though. My question is: how can I add external attendees to the meeting I'm creating? Is there anything specific around attendees?

After a few exchanges on the Microsoft Skype for Business MVP's private distribution list, it appears that attendees have to be part of the organization or otherwise the call will fail.
Submitted a Pull Request to update the latest version of the documentation