I can't remember any! There was "Conclave of Darklord" by Nobuaki Washio that featured 6 simultaneous players ( https://www.youtube.com/watch?v=wLtoag0rD50 ), but I do no remember any with 8!
(Apologies for the late reply, and shameless self-promotion ) Stracon 2 supported up to 8 players by establishing a MIDI link between 2 Music Modules.
I also saw a similar sort of setup on one of the Dutch MSX fairs once, where someone made a multiplayer Tetris? I can't remember the name of the game, but also supported quite a lot of players I think.
The Tetris game you mean is Triplex and can be played with up to 16 players, when connecting MSX computers via MIDI cables using a Philips Music Module.
I don't know any MSX games that support 8 player but for new developments it suits well with collective sports like Volleyball and Dodgeball, or even an 8 track running game.
And there is a homebrew game from Atari 2600 named Fall Down that can be escalated to multiple players. Yet on Atari 2600 there's an 8 player game named Vroom which looks like a slot car game.