This is a very simple SourceMod plugin which adds a single command: sm_player_teams. It prints a list of all the player's Steam IDs, and a number indicating which team they are on, in the following format:
<steam_id>;<team>
Where <team> is 2 for Terrorists and 3 for Counter-Terrorists.
This was created for use in one of my cogs for Red-DiscordBot, retakechannels.
To install, simply put getteams.smx in csgo/addons/sourcemod/plugins/.