Thanks for the reply to this.
I reviewed the links, but the issue is that the mail-enabled security group that I want to be automatically added for any synced AD user, already exist in the environment.
Plus I can't change it's 'Membership type' to be Dynamic User. But that is for a security or standard group. I think any mail-enabled security group must be managed through the Exchange admin center. They can't be managed through Azure AD which is why its not supported on Graph yet.
Hence, on the Exchange Admin Center, I don't know if a dynamic distribution group can be mapped to a mail-enabled security group. Because again, this mail-enabled security group already exist.
Any thoughts on that? Or is there something that I'm missing here.
Thank you!