If you want the the plot to be sorted by the Response variable value you use the option CATEGORYORDER with either RESPASC or RESPDESC to order in response variable ascending or descending order respectively.
proc sgplot data=sorted_invites_summary;
hbar City / response=city_count categoryorder=respdesc;
xaxis label="City";
yaxis label="Number of People Invited";
run;
Note that for many of the graph types sorting the data doesn't really translate to order in the graph unless there is a DATA option for an some sort of Order related option.