Adding new circuit breaker, no ground posts

My electrical box has 4 spaces open for circuit breakers and I am adding a new 20amp circuit for a basement outlet and light for my computer. I have the permit and are working on the rough in. The service is 200 amp and the house is 10 years old. But when I look at the ground posts there are none open. I don't know if some CBs use more than one or what or why they are all used especially when there are 4 circuit breaker slots open. Can I double up ground wires in the ground post holes? It looks like the ground post has a slot the wire goes in and you tighten a screw to secure the ground wire. The hole is big enough to put two in there but I want to know if that is OK and legit. If not what are my choices. I really don't want to add a new breaker box for one new circuit. Thanks for any help!!!
Sheila
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If this is the main circuit breaker panel and not a sub panel, you can double up on the ground (bare) wires, but not the neutral (white) wires. You can also buy another grounding block and add it to the panel

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