You need to use Status in the console in-game to get their U:XXXXXXX ID. You can then enter that into either https://steamrep.com/ ,http://steamidfinder.com/ , or https://steamid.io/

Status command [U:X:XXXXXXXX] or Steam profile link. Run in rep.tf (shortest url) and find the STEAM2 link.

da fuuuuu

I personally use https://steamid.io/
How this works: If you have their profile page url, you can copy and paste this into the little search bar on steamid.io. This will output the player's Steam ID, along with their username etc etc.

I'm not sure this is available for regular players, but using sm_steamid <name player>(in console), or /steamid <player> would also grab their SteamID.
Aside of this, to get people profiles on Steam, I have it set in my Advanced settings that when I hold the TAB list of players, I can right click a player and it'll give me a few options. One of these options is opening a new tab on the browser to their profile. Doing this however, prevents me of interacting with the game until I no longer hold TAB. I'm not exactly where you can enable this, but it should be in Advanced settings in in-game hud.

If any of these things are still unclear, tell me so I can try again.

Hm... I've skimmed through your reports thanks to your profile... The problem here is you 'couldn't get the steamID in time'. So the solution is a bit of a tedious task but you'll have to type in 'status' in the console (in advance) whenever another player joins the server (preferably the 'new' ones).

It really is tedious but you'll have to bear with it. However, once you finally got the 'steamID' you'll need to paste it on either one of the websites Cammy have mentioned (I prefer http://steamidfinder.com/). You can paste it on the white bar (this is from the website I mentioned) and click on the gray button that says "Get SteamId"

Once you have done that you'll have to look for this:
(I will be using my steam ID for this EXAMPLE)

steamID: STEAM_0:1:58184349 <--------------- This is the type of steam ID what the admins want/like/prefer in the reports.

Or you can try the alternative Lolboy mentioned and type in sm_steamid <name player> or /steamid <player> in the console. I do not know if it works, I might try it sooner or later.

We have a custom command for that: sm_steamid <name|#userid>