If your pickup's baseplate is grounded, then the metal ring should be grounded simply from the contact with the height adjustment screws and springs. If you need a more secure ground connection, then just take some copper shielding tape and run a strip of it inside the
pickup cavity, up the side wall, then over to one of the screw hole areas. Solder a ground wire to the tape in the cavity, then when you screw the mounting ring down it will make a hard connection. Make sure everything is clean.
But seriously, the Invader's baseplate is connected to the shielding in the pickup wire. There should be almost no way your metal ring isn't grounded right now, unless something isn't grounded properly inside the control cavity. If you have a multimeter, just touch one end to the ring and the other to the output jack shell and see if you're connected.
