I am adding a sub panel to my garage.
The main panel ground and neutral Bars in the main are full.
Since they are bonded, can I just add a new ground bar In the main panel and run both the ground and neutral off of the new ground bar for the sub panel in the garage ?
You can have as many ground bars as you want to add.
However neutrals need to be on the provided or factory authorized neutral bars.
On a ground bar, the only current path is the screw threads mounting the ground bar to the panel. That is fine for grounds, which only carry current during ground faults. But neutrals carry current all the time, so they have to worry about reliability and conductor heating, arcing, etc. The factory neutral current paths are engineered for this.
Make more room on the neutral bars by evicting some grounds over to the new ground bar.
It’s unheard of for a panel maker to provide fewer neutral slots than the panel can legally use. For instance if you have a 20/28 panel that allows 8 spaces to be tandems, there’ll be at least 28 neutral spaces. But there might not be 40, so if someone cheated a way to fit tandems in every space, they could run out of neutrals.