How would I prevent the party board from allowing multiple players?

I was planning to make a party system to teleport 4 players that join into a reserved server, but the first thing to start is to make a party system allowing players to fight together with the boss.

However, I'm not sure how what method should be done to allow players to only join once or so.

-- Join Party
Board = script.Parent.Parent.Board
PartyPlayers = Board:WaitForChild('PartyPlayers')

script.Parent.ClickDetector.MouseClick:Connect(function(player)
for i,v in pairs(PartyPlayers:GetChildren()) do
    if v.Value == player then
        else
    end
end
    if PartyPlayers.Player1.Value == nil then
    PartyPlayers.Player1.Value = player

    elseif  PartyPlayers.Player2.Value == nil then
    PartyPlayers.Player2.Value = player

    elseif  PartyPlayers.Player3.Value == nil then
    PartyPlayers.Player3.Value = player

    elseif  PartyPlayers.Player4.Value == nil then
    PartyPlayers.Player4.Value = player

    end
end)

-- Leave Party
Board = script.Parent.Parent.Board
PartyPlayers = Board:WaitForChild('PartyPlayers')

script.Parent.ClickDetector.MouseClick:Connect(function(player)

for i,v in pairs(PartyPlayers:GetChildren()) do
    if v.Value == player then
        v.Value = nil
    end

    end
end)