My outlets have ground wires twisted together and bonded to the metal box, but not the outlet. Is this ok?
Ground wires were not implemented on a large scale until a few decades ago (possibly after your home was built). Outlets themselves may not have even had ground screws. 

You should definitely add pigtails to the existing connection and ground your outlets. Use the same size wire as the existing ground conductor (or slightly larger if that's what you have on hand). 

In case you're new to house wiring, be sure to 1) create good loops and wrap them clockwise on the screw, and 2) not put more than one wire under any screw.