I am lead to believe that the matchmaking system in League of Legends matches you against opponents based on summoner level. However, in the case where you have a pre-game lobby with players of different summoner levels, how is your lobby matched again opponents? Is it based on highest summoner level, lowest, average?
Zileas, design director for LoL, explains:
You can read the full shebang here. Basically, a custom ELO derivative is used to rank players and power the matchmaking system, so that you are paired with similarly ranked teams.