All grounds in a box are connected together (and to the box). So attaching your new ground can be to any of the other grounds. Ground wires are either green or bare.

It looks like there may be another bare wire to the right that isn't snipped off (see blue circle in picture below). If there's enough slack, you may be able to just sand off the paint and then use a wirenut to attach your new green wire to that bare one and leave all of the screws as they are. With all that paint in the box, it may be difficult to tell if you get a good metal-to-metal connection if you move anything to do with the screws.

Canadian plastic boxes usually have a ground strap that reaches up to the yoke mounting screws. This is one of those boxes. You can land a ground wire on any ground terminal that is available. You can only have one wire under the screw so if you need to connect more than one it is admissible to use a single wire under the screw and use a marrette (now I'm talking Canadian, eh?) to joint the wires.

Hopefully there's not paint under the screw but if there is, as David says, scrape it off.

Why would anyone spray paint into an electrical box?

They spray paint the walls and it gets into the boxes because the devices and plates aren't installed yet and the boxes aren't covered.

Is there a reason why you can only have 1 wire under the screw? Is it more to avoid a wire becoming loose, or is there any risk/harm?

It's to avoid a wire from becoming loose which IS a risk/harm.

The ground wire is a safety feature that isn't used until there's a problem. So you won't know that it's become disconnected until something bad happens.

Greg is right, further, the terminal in your box is specifically designed for a single wire.

"the terminal in your box is specifically designed for a single wire."

I suppose it's technically still a violation, but if the screw is long enough, crimping a properly sized ring terminal onto the wire effectively eliminates any real negative effect on the connection quality when you add another wire to the ground screw.

I'm not sure I'd advocate it for others, but I wouldn't hesitate to do this myself.

PS: Crimping works best with stranded wire. With solid wire, you really should solder, too.

Nobody said anything about ring terminals. We were talking about bare wires under the screw. If it were just a screw, you could put three ring terminals under it.

> Nobody said anything about ring terminals.

Not so. I did. :)