I'm trying to replace a single pole light switch with a Leviton 6674 dimmer. This dimmer has four wires including the ground wire and a wire which is tagged and used for three-way applications which I won't be using. The problem is the old light switch did not have a ground wire attached nor do I see one loose in the metal enclosure box. I've been searching online for a similar wiring setup as this but can't seem to find it. I am just wondering how to properly install and ground this dimmer. The box enclosure is metal and I noticed two screws inside and at the back of the box with three different bare wires attached to these screws. I assume I will have to attach the ground wire from the dimmer to one of these screws. The left screw has one bare wire attached that originates from the top left corner of the back of the metal enclosure while the right screw has two bare wires attached that originate from the top right and bottom right of the back of the metal enclosure. It's difficult to see the wiring setup with the photos I took and the paint discolouring some of the wires so I included a rough drawing of the setup to help visualize it. I would include more photos but I am limited to two. The home in which this fixture is located is nine years old so I would like to assume that they wired it to fairly recent standards. Any help would be greatly appreciated! Thanks!
If it were me, I'd take all the grounding wires out from under the screws. Then I'd collect up all the grounding wires, including the one from the new dimmer, and an extra bit (6" or so) of spare bare wire of the same size. Using a twist-on wire connector, I'd connect all the grounds together. Finally I'd slip the other end of the short bit of wire (pigtail), under one of the screws in the back of the box.
This way all the grounds are bonded, and you're not relying on the metal box as a fault current path.