Any negative (-) wire, is 'ground', on the bike. The negative (-) terminal on the battery, is connected to the chassis, which is dubbed, 'ground'. Yes, you can bundle all negative (-) wires, and then connect them to the chassis, or another ground wire (every connector introduces resistance, so connecting the bundle to the chassis, is the better option, introducing less resistance, but it is negligible, most of the time -- soldering is recommended, over wire nuts, and twisting the wires together and covering them with tape). Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII