As usual, connect all ground wires coming into the box. Use an additional short length (pigtail) to reach each receptacle or switch green ground screw.
Method 1. Connect ground wire to receptacle or switch as usual. Get a special clip (sold separately for that purpose) to squash between the tab of the receptacle yoke and the box as you screw the receptacle in place. One clip per box.
Method 2. Use an additional bare pigtail to connect to a screw into the box (or to a special clip that fits over the edge of the box). Note that the screws that hold cable clamps at the back of the box cannot be used because they can't be tightened enough without damaging the cables.
Method 3. Instead of connecting ground wires to the receptacles, connect just one ground wire to the box. Use the special clips, one per receptacle (and switch) when installing these in the box.
You need 12 gauge ground pigtails for a 12 gauge 20 amp circuit.
Using green wire nuts with a hole in the small end, one of the incoming ground wires can be left longer and go through the small hole and then be connected to something, so one fewer pigtail is needed. Two ground wires cannot go through the small hole because screwing on the wire nut will chew up the wires.
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