If you're not using a Slingbox M1:
Slingbox 350/Slingbox 500/Sling TV: Slingbox.com - Giving a guest user access to your Slingbox 350 or Slingbox 500
Models of Slingbox before the 350/500/Sling TV: Slingbox.com - Creating a guest account and Slingbox password
How the guest connects to the Slingbox is up to them. They might use a computer or mobile device.