How to write a PowerShell 2 script to see what users are logged on?

I’m learning PowerShell. I have a test pc set up with Active Directory, called Enterprise1 (for Windows 2003 Enterprise edition sp2).
How would I write a PowerShell 2 script to see what users are logged on? Also how would I write a script to send all logged on users a message then log all users off, like in an emergency situation?