I have an IntValue object named Que that I want reachable and editable by all clients. If someone joins, it only updates their Que value and the other players don’t see it. The Que is to start the game and if it stays at one, the game won’t start. BTW it's in ReplicatedStorage right now.
I'm still trying to figure this stuff out myself, but I think what you might want is a localscript in player's startingscripts that will have a FireServer event and a script somewhere on the server that has a OnServerEvent that actually updates your value. That way, when a player joins it will send the message to the server to update the value.