I'm trying to figure out exactly how to use Greenie grounding connectors. In a 2 or 3 gang metal box, can I join each receptacle's ground wire and then fasten the single wire coming out the tip of the Greenie to the metal box with a grounding screw. Is this an acceptable way to ground multiple receptacles using a Greenie?
The green wire nuts made for grounding are not a NEC requirement, and they're really not that special. The hole in the end is the key feature. There's a number of ways you can use them, and it's really just a matter of what you feel makes life easy and makes a solid install.
If you leave one of the ground wires long, you can splice all the ground wires coming into the box, run the long one through the hole, and hit all the devices with that one.
What if you didn't leave one long? You could make a big long bare wire pigtail, terminate one end on the ground screw, then run it through the hole in the greenie. Then twist and splice all the ground wires coming into the box with the greenie, then use the other end of the pigtail to hit every device. If you have a massive box with a zillion ground wires you could make an even longer bare pigtail, and thread multiple greenies on it.
Most of what you can do with greenies, you can do with crimp rings. Crimp rings are cheaper and take up less space, but greenies are much easier to undo and redo.
Green wirenuts with a hole in the tip are stupid.
You're not supposed to stick one of the wires in the top. You're supposed to stick all the wires in the bottom, and one of the wires is 9" longer than the others and is supposed to stick through to let you ground one more thing.
What does this mean? You're supposed to strip about 18" of Romex sheath off, and then cut 9" off all wires but the ground wire. So you are wasting nearly a foot of Romex. The logic is you are supposed to use the cutoff wires as pigtails later. Meh.
What is pictured is a perfect use case for it, though, as it already has 6 ground wires incoming and 3 switches to also deliver grounds to. Without tricks, you'd need to put 9 grounds under a wirenut, (plus one for the box). They don't make 10-wire nuts, so you'd need to be split into 2-3 wirenuts with a pigtail between them. This latter approach is exactly what I would do.