Use VCE and minigame events. You can use onMinigameKill to increase a players "points" and then a buy area at spawn where players can "unlock" weapons by clicking on them which sets another variable causing said weapon to be added to their inventories when they spawn at the battlefield.
However, I would advise against this as the end result is the original joiners killing all of the new people that join, then the original people leave because there's no one to kill and then the game is just a fail.
Also, I'm not telling you the exact events and setup as this is something that you really should learn as opposed to being told how to do every step.
