From Liquipedia StarCraft 2 Wiki. Individual teammates have very little effect on the team rating, except to determine what placement matches they are put in. There are numerous divisions within each League, with each division being composed of up to players. For example, a player in Silver Tier 1 would be closer to a promotion into Gold than a player in Silver Tier 2 or Tier 3. These aren't quite ready to be revealed yet, but are definitely worth the wait.
The following section contains information from a previous version of StarCraft II which is no longer valid.
League (StarCraft II)
List of StarCraft II units. For 3v3 and 4v4 games, only four skins can be set per race. The player is given simple goals against an AI opponent.