The simple way to see who is running the prg is to have a user table and flag each user as active when they are using the prg.
For running only one instance see Mutex http://fox.wikis.com/wc.dll?Wiki~ControllingTheNumberOfApplicationInstances.

>Dear all,
>What is the best way to know which users is running the program?
>I have a MyApp.exe that is in MyApp ( a shared folder ) on my server. I have 20 users that i want to know which user run the program and each user must can only run the program once.
>Thanks