How do you add a azure ad user to a remote desktop users group?

For on-premises or Windows Virtual Desktops?

on premise

I must admit that I've not been successful at it as of yet for any of our on-premises Remote Desktop deployments. We need a local AD.

For one computer or many?

Probably a better way but you can login with account and you will get the SID.

Use a script such as  https://gallery.technet.microsoft.com/scriptcenter/5dc1e9f8-ec28-4f2e-aeea-65f6589afac5 to add SID to Remote Desktop Users

It's just for one computer.

My team will play around with the scripting solution some more and post some updates later.