The only easy way to connect to a remote ubuntu by different users, and each user having his own desktop session, is to:
- Enable XDMCP in the GDM config on the remote PC;
- And in the workstation that will connect to it, you must logout your desktop session in order to connect by the XDMCP in the GDM login screen.
There's also the VNC-way, but you must type commands and edit files in order to do so.
There should be an easy way to remotely access the GDM screen in a window. For example, Terminal Server Client (tsclient- has XDMCP support but it is not enable by default, because Xnest is not installed by default. It should be installed "out of the box"

Or maybe add Nomachine NX free edition into the commercial repository ? That is a nice remote desktop too, and is secured by default, which is not the case for VNC and XDMCP