PopTrayU, by default, saves all its settings in the PopTrayU directory and would be shared by all users.
You can specify a different directory for each user by using the command-line:
"C:\Program Files\PopTrayU\PopTrayU.exe" "%USERPROFILE%"
"C:\Program Files\PopTrayU\PopTrayU.exe" "%HOMEDRIVE%%HOMEPATH%"
The settings files will then be created in each users home directory,
which is different for each user on the PC.