I am using a plex server in my home. For music I installed a plexamp headless on a pi.
With my account (admin) I can cast directly form my phone (ios) to the plexamp headless connected to my hifi system.
I create an account for my wife and she can enjoy plexamp with her account on her phone (android). But it’s not possible to choose the plexamp headless caster, it’s not visible in her account.