I think the OP is under the impression that each IO pin should have its own ground pin to connect to.
That is not how it works. All the ground pins on the board link to the same lump of metal. You can connect to any place on the board that is part of that lump of metal - the ground pins, the outside of the USB connector, the negative connection of the barrel jack - they are all the same.
You can connect as many wires as you like to one ground pin.