How can I add users to the Remote-Desktop-User Group in Windows Home OS?

You don't have the option "Users and Groups" on a Windows Home  resp. Windows Standard operating systems.

However, on any Windows Home /Standard (Windows XP Home, Vista Home Windows 7 Home, Windows 8/8.1/10/11 Standard) it is not required (not possible) to do this configuration step !

By default administrators and standard users have the permission to log-in via RDP to Windows Home systems with XP/VS Server installed.

If the problem still resists, first please check if you have installed XP/VS Server version latest version.
If not please update.

If the problem still persits upload a TSRP (Thinstuff Support Request Package)

C:\Program Files\Thinstuff\XPVS Server\TSSupportPackage.exe

on that computer where XP/VS Termial Server is installed and follow the instructions to upload the package.

We will then analyse it to find the problem.