It's not clear to me why ground wires aren't subject to the same rules as neutral wires (one per bus bar hole). What's the reasoning here?
Electrical – Why is it OK to put multiple ground wires in the same bus hole in an electrical panel
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Related Solutions
Mixing Grounded and Grounding
If this panel serves as the main disconnect, and the grounding and neutral bus bars are bonded. Then yes, you can terminate both grounded (neutral) and grounding conductors at the same bus bar.
Notice that the bus bars are bonded using a bonding strap.
Multiple Conductor Terminals
If the bus bar is able to accept multiple conductors under some or all of the terminal screws, it should be labeled as such either on the bar itself or in the panel documentation. Terminating multiple conductors under a screw that is not rated for it, can cause bad connections, excess heating, damage to the conductors, etc.
Additional Bus Bars
Some panels are configured to allow additional bus bars to be installed. If your panel is one of them, you could install an additional bus bar and terminate extra conductors there. Other panels may accommodate extended (longer/larger) bus bars, that can be installed in place of the existing bus bars.
Seeing that this is your main panel the ground bar, neutral bar and the metal of panel should be bounded, usually with a bonding screw, in some way. So putting the new ground wires on the neutral bar is okay. Now with that said, if you don't want to do that, why not add another ground bar if you have the space? If you look at the sides, top or bottom of the panel you may see holes (with a ground symbol) for another ground bar to screw down to. If you see this turn off the power and add the bar.
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Best Answer
Grounding conductors are designed to only carry fault current, which is typically a very short burst of very high current.
Neutral conductors carry circuit current, which means they go through heat/cool expansion/contraction cycles. Multiple neutrals under one screw can lead to loose terminals due to different rates and levels of expansion and contraction.