If a player is not in group it makes a instance message into workspace?

I'm not exactly sure what you mean, but I have made a script for you that may or may not do what you want... Here you go.

To add your group, get the group ID and replace the 000000 with the new group ID...

g={000000} -- Group ID

function check_groupStatus(player) 
    inGroup = false 
    for i,p in pairs(g) do 
        if player:IsInGroup(p) then 
            inGroup = true 
        end 
    end 
    return inGroup 
end 
game.Players.PlayerAdded:connect(function(player) 
    if not check_groupStatus(player) then 
        local h = Instance.new ("Message")
        h.Parent = game.Workspace
        h.Text = (player.Name.. " is not in the group.") -- A public message that says this player isn't in the group.
        wait(6)
        h:remove () 
    end 
end) 

Hope this helped!