Why do you need a new ground bar? Is there no ground bar now? The neutral buss in service equipment is also the ground buss. In panels that are not serving as service equipment the grounded current carrying conductors (neutrals) and the Equipment Grounding Conductors, or EGCs, must be kept separate and insulated from each other to avoid the current in the grounded conductor from taking stray paths that can cause a shock hazard or arcing. I take it from your previous postings that the panel is not the service equipment and that you have a separate disconnect feeding it. If I'M right read on if not supply the correct layout.
If you are going to run just one new circuit from a panel, that is not service equipment, that has only served raceway or metal jacket grounding paths in the past then you can just use a metal cable connector and a half inch bonding bushing to connect the Equipment Grounding Conductor to the grounded metal Cabinet of the panel. If you will be running multiple circuits with separate EGCs from that panel you should install a ground bar.