Alternatives to pig-tailing ground wires in outlet box?
I would use a crimp sleeve to save space.  Hold the ends of the ground wires together side by side so that one's about an inch longer than the other and crimp the sleeve on at the end of the shorter wire.  Loop the end, land it on the ground terminal, and you're all set.   

Follow the instructions on the package of the crimp sleeves.  Most people use linemans pliers with a crimp lug to crimp sleeves.