First launch the game, but don't go to a server.  Go to Options, Keyboard, Advanced and then click "Enable Console" and hit apply or okay.  This will let you hit the " ~ " key, which on my comp is next to the "1" key.  In-game that button will bring down the console.

Then you enter "status" and hit submit.  It will list all the players and their Steam IDs.  Highlight and right-click on the highlighted item and choose copy.  You can then hit ALT+TAB to get out of the game, but leave it running in the background while you paste that into your word processor, or wordpad program, which you've left open for that reason and do a right-click and paste.

I usually jump on and hit status so I have everyone's ID in case there's some funny business later and the person logs off before you can capture his ID.  This happens often.  I usually end up not reporting because if I did that, I would spend my time doing that instead of playing. And folks complain that I don't play - because I'm policing the damn server. 

You're better off X-Firing an admin.  Never one on-line when you need one it seems.

EDIT: What Blood said. He beat me to the punch.  Record date, time, map and server. If you want, you can hit "F5" and take a picture of the incident to post with your report.  -Usually a warning about what the server rules are and a please stop makes people behave.  Unless they're drunk.

You should be more like Cantrip.



Twitchy: "I pulled a Cantrip . . ."
GivePeaceAChance: "If the Tempest had nipples, I'd pinch that shit."
Xylicon: "I'm about to deliver my [Richard] in your face for posting that stupid shit. "
WDF?: "He keeps reaming me... night after night... I'm rather sore."
GivePeaceAChance: "OMG MY NIPS ARE HARD"
llenta: "cantrip, you fail!!!"
Filliam H. Muffman: "i'm on top of dick"