I'm not sure about this command but if you do /party (player) do they have to accept or you just can't hit them?
I'm not sure you fully understand. /party is part of mcmmo, there would need to be a custom plugin for that. Plus, if you are suggesting a plugin, cyp asks for the plugin page if you read his suggestions post.
I said that they use a custom plugin, plus, mineplex is much more advance and has many more custom plugins, /party does a completely different thing. /party to stop you hurting others in your party is originally mcmmo. Developers have made custom plugins, plus if you want a plugin, cyp said put the plugin link, Cyp won't be bothered looking for it...