Right way to add standard receptacle to this GFCI circuit
You can use twist-on wire connectors (or other approved connector), to create a "pigtail".  


Turn off the power.
Remove the LOAD side wires from the GFCI.
Using a twist-on wire connector, connect the white wire you removed from the GFCI with the white wire feeding your new receptacle and a short bit of the same size wire (6-8"  should do).
Do the same with the black wires.
Attach the bare ground wire from the new cable, with all the other grounds in the box.
Connect the other ends of the short bit of wire to the appropriate screw terminals on the GFCI. 
Reinstall the GFCI. 
Turn on the power.