I dont know if you really need a custom command for this, nor do i know of a customapi to help you (though im sure there is one). I could imagine you could simply run a giveaway where the keyword is !join or something and you randomly pick from the people who have used that keyword. If you want a queue system of sorts there appears to be this custom api CustomAPI to manage queues though i haven’t used it personally
But i dont know of a command for multiple parties and choosing from them, seems a bit more complicated than it needs to be