How do you make a cmd for players to use? (!rejoin)

You can use the .Chatted event from Player.

Try something like this:

local Players = game:GetService("Players")

local function playerAdded(Player)
	Player.Chatted:Connect(function(Message)
		if Message == "!rejoin" then
			-- Rest of code:
			-- Use TeleportService to have the player rejoin the game.
			-- https://developer.roblox.com/en-us/api-reference/class/TeleportService
		end
	end)
end
Players.PlayerAdded:Connect(playerAdded)