Beginnig with Server 2012, it is no longer possible to setup Security for RDP-Tcp with a GUI. Here is the WMI Command:

wmic /NameSpace:\root\cimv2\TerminalServices PATH WIN32_TSPermissionsSetting.TerminalName=“RDP-TCP“ call AddAccount „DOMANIN\GROUP_OR_USER“,X

Where X is :

0 = WINSTATION_GUEST_ACCESS
1 = WINSTATION_USER_ACCESS
2 = WINSTATION_ALL_ACCESS

To show already configured security:
wmic /NameSpace:\root\cimv2\TerminalServices PATH WIN32_TSPermissionsSetting.TerminalName=“RDP-TCP“ get StringSecurityDescriptor