My favorite game ever, age of empires 2, uses an ELO system with multiple players (up to 8 players). I don't know how the hell it does it and I myself have also been looking online for a system that applies for something more complex than 1 on 1 games like chess.
Try looking into LoL and DOTA 2 rating systems maybe as they may be pretty complex and you can maybe make a couple of changes to that to find the method you're looking for.
Try looking into LoL and DOTA 2 rating systems maybe as they may be pretty complex and you can maybe make a couple of changes to that to find the method you're looking for.