What is it?
An optional way to organize players in a multiplayer game into teams. This can be used for things like unofficial competitions with players on the same server or to allow private communication between a group of players in a free for all game using Chat. There might also be other uses that we haven’t thought of so be creative! This feature was added to theHunter on August 29, 2013. 
How do I use it?
Teams can be administered in the game via the “Teams” menu in the full screen interface (ESC).
As a client you can choose to join/leave teams using this screen if the host allows it.
As host you have a wide variety of different options to manage team games such as the ability to move players between teams, a round timer and private teams. Each option comes with an instructive tool-tip which should be able to give you an idea of its use.
Quick Score Screen
The quick score screen can be used by any player in a game to check the current team/player scores within the game. This screen can be toggled by pressing the F3 key and is, in contrast to the team screen, not viewed in fullscreen which allows players to be aware of what's going on in the game while viewing it. This screen is also automatically displayed to all players when a round ends (how this happens is up to the host) which allows all players to see the final scores before they are reset for the next round. On September 12, 2013 the player's HunterScore was added to this screen. 
Players can send messages to the team by starting a chat message with “/g”. When doing this the message is only received by members of the same team. This can be useful in competitive situations when you don’t want other teams to know your plans. Team messages have a different color than regular messages which all players can see (it is blue/green).
While in chat, you can quickly toggle between general chat and team chat by pressing the TAB key on your keyboard before you start typing your message. You can see the current mode of the chat at the prompt - the '[A]' means the text you type will be visible by everyone, while the '[T]' indicates that the message will be seen only by the members of your team, if you are currently in one.
What about leaderboards, stats etc.?
What happens in team games stays in team games. CSS scores and other team related stats are currently not stored on theHunter game server. This means that there are currently no team related leaderboards, stats etc. available on the webpage to be viewed once the game ends. Normal stats are still sent to the server, so you can still get achievements, get on the regular leaderboard etc. while playing in a team.