How do I make a GUI pop up only when people are out of the Lobby and playing the game? [closed]

Use teams and ondied event. Like this

game:GetService('Players').PlayerAdded:connect(function(player)--finds the player
    player.CharacterAdded:connect(function(character)--activates the function
        character:WaitForChild("Humanoid").Died:connect(function()
            print(player.Name .. " has died!")
        end)
    end)
end)

For more info check this out http://wiki.roblox.com/index.php/Died_(Event)

Also, you want to switch the teams if they died, so on the died event, you change the team.