1- Have and know how to use cheat engine.

2- At the main menu press Start Game, not Find Game.

3 - Find the value of max player.

Lets say you set it to 15, In Cheat Engine first scan 15. Back in game, change the lobby value to 28, back in Cheat Engine, then next scan for 28. Eventually you will find the address or addresses then change the number in Cheat Engine to whatever you want the size to be.

I personally do 78 as max limit because that's how many beds are in the lobby. Be aware the more people there are the more likely lag, crashing, and such problems will appear. But if you don't go to crazy it can be hella fun.