There's basically two or three things you need to know:
When making a multiplayer game, the startscript is used for server initialization ONLY. Use the player startscript for player init. If you have QuestNet installed, there are some additions to the QDK Game Properties that will do this.
If you want to involve specific players rather than the entire game, use
userid.
Examples:
health[userid] will change the health for the userid that activates it (Read Manual)
$name(%userid%)$ will show the player name for that userid (Read Manual)
$name and $id are new as well.
READ THE MANUAL! It is invaluable. I use it ALL the time.
If you don't have a copy of it, the link is here:
http://www.axeuk.com/quest/developer/asl/manual.htm