Can you attach more than one wire in a given hole on a bus in an electrical panel?
The issue isn’t about mixing ground and neutral on a single bus in the main panel. That is allowed although it’s preferred to have separate neutral and ground busses, both bonded to the chassis and grounded.

The issue is that most panel busses aren’t rated to have more than one wire per termination point. Some are but most aren’t. The issue, other than just the question of certification, is that they may just not be designed to hold two wires securely or to keep a tight connection over time.

If you can post a photo of your panel, or at least give the brand and model, someone here should be able to give you a specific answer.