Turning no playerkilling off

[excaliber7388]

How can you turn off noplayerkilling in a level on clientside, so it's only for a few players. It's for a spar, but I'm having trouble with making it so only 2 people can enter, and disabling the nopk zone. I know other servers do it, but I can't figure it out.
Basically I need a script to turn off the no pk (without serverwarp)
and an idea for making a script that allows only 2 people in a spar