Get usergroup by SteamID

I am looking for a way that I can find a players usergroup by their SteamID. I want to be able to get their group even when their not online which is causing a bit of confusion. Basically, I found that in my garrysmod/data/ulib/users.txt, it puts SteamID’s and relates them to a group. Is there a way I can parse this file to find out which group a certain SteamID belongs to?

Ah ok. So then would I just parse it as an array/table?

No, it returns a file object: http://wiki.garrysmod.com/page/Category:File

Alternatively, you can use ULib.ucl.users


for steamID, data in pairs(ULib.ucl.users) do 
    if steamID == "STEAM_0:..." then
        ...
    end
end