Grounding in main panel without visible ground wire or ground bar
This panel is all done in metal conduit.  There are no ground wires because the conduit serves that purpose. 

When in Rome, do as the Romans do.  Whoever went to all the trouble to use metal conduit, probably did it because that is a Code requirement in your locality. So continue onward in same vein.  Fit metal conduit e.g. into a knockout in this panel.  Run the conduit to your new recep location. Use a metal recep box.  Then throw two THHN wires into the pipe - white/gray for neutral, and any other color for hot.  

As far as landing that wire on a breaker, just pick a breaker whose outlets are is unlikely to be used hard while the outdoor recep is.  Then pull the hot wire off its breaker, put a pigtail on the breaker, and join its original wire, and your new one with the pigtail.