If you dont know how to open your console read this first
clickie Now this is very simple to, all you need to do is type Status is console and you will get a list of all the players on the server that looks like this:
# 459 "[ZMoD]spelworm" STEAM_0:X:1077XXXX 1:47:25 27 0 active 84.105.XX.XX:27005
now all you need to copy is this STEAM_0:X:1077XXXX
(note: all the XX means I removed the numbers there)
Portal 
