is it OK to share a grounding bus hole between two wires coming in? thanks

Here's my problem. I was installing a GCFI and fixing some of the bad wiring in my garage the other day, and realized that half the receptacles were grounded and half weren't. I opened the ungrounded receptacles, and they all had grounding wires attached, so I followed them back to a junction...

Ok...so I have a dumb, newbie question. Is it allowable (or advisable) to run a ground wire from an outlet panel to the metal junction box? I always was under the impression that you needed a ground that run to a grounding rod or metal pipe. I have a house built in 1915 which doesn't have...