I notice a lot of people playing Halo 3 online have the same user name 2 or 3 times....but with a (1) or (2) after the name. Are they playing splitscreen, and if so how do they setup the other accounts?
They're using guest accounts. Plug in a second controller and it'll give a sign in option for that pad. Guest will appear underneath all the gamertags on that console and it copies the console's primary gamertag for its name as you have described once you've selected it. It doesnt save any info, cannot earn achievements and vanishes in the aether upon signing out with that controller, so its fine for when you have a mate or two around, but is hardly ideal for long-lasting campaigns.