Make DAILY missions for 5-man parties and give half the rewards each 5-man game!

Encourage people to play in large groups!!!

Much how MMO's reward those who participate in raids, dungeons and PvP, playing as a 5-man party should feel rewarding to all those who take the time and energy to do so. There is effort behind organizing and playing in large groups and new friends can be met (AKA new reasons to play for most players!) for those who would otherwise just play solo queue and meet other players mid-match. Similar to daily win missions, there should be 5-man daily win missions for anyone who plays with others as well as a portion of that reward given regardless of wins or losses.

Make them sweet rewards as well! Most people won't complain about recieving Little Legends rewards, a substantial amount of BE/Orange Essence or a choice between a chest or key. Random skins - perhaps even legacy skins - would make for a sweet occasional awards for weekends too in order to encourage more people to socialize when playing LoL and meet new amiable players!

Giving those willing to 5-man more incentive will make for better queues as well! Fewer friends will mess around as duos in ranked soloq if they can instead cooperate with others and earn sweet stuff! 5-man missions might even make regions better overall!!!