Instead of adding the 6th player, he would be shotcaller/coach. He could use F1-5 to cycle all players, ping, advise ganks, and instead of abilities he could have arrow waypointer, target marking enemy champions, impersonated shop buy calls. Instead of mini version tab he gets one where he can see gold and exp of all teammates.
Alternatively, (s)he can be courier. Meaning that (s)he has the ability to carry item(s) to players and maybe even have a trinklet slot. In lategame when all are full build he can carry vision wards for team.