thegreatdamarmu,
I have hacked that exact pcb and it works fine and should work for your needs. If I remember correctly it is common ground which means you can "daisy Chain" your ground wires. Look at the pcb carefully, each button location has two semicircles, when the button presses down it forms a connection between these two semicircles which indicates to the pcb that you have pressed that button. Follow the traces from the semicircles. Notice if the traces are shared from one side of each circle. If they are, then it is common ground and this will also indicate which side is the ground.
Just in case: first put a dab of solder on the pad (semicircle) and another dab of solder on the wire, then put the wire in place (on the semicircle) and touch it with your iron. You'll get a good connection this way. After you test everything and it works, secure all your solder spots with hot glue.