I have several outdoor Bonsai and I am not sure what to do with them when the ground freezes. I am aware that the trees need a time of dorment so I can't bring them indoors so I simply leave them to cope as they are?

Without knowing your location, climate and species it is impossible to answer.

Each tree species has a range of termperatures that are safe to handle. All my frost-hardi species are left to deal with frost, but I never get frost that is denagerous to my trees. SO it is important to understand which species and which climate you have to really help you.