REST API: Adding remote user to a roster group

Does anyone know if there’s a way to add a remote user to a roster group through the REST API?

Trying to add a user to a group through the REST API requires the username being in the Path, which I’m assuming means that user already has to be on the server. And also because I tried doing it that way already.

Am I assuming correctly that there’s no API endpoint where the group name comes before the username?

Thanks in advance!

— Kevin