Are karate gradings done on your own or in groups?

It depends entirely how your school does things. I tend to grade the younger kids in groups, it reduces anxiety, freak outs and tears.

In most cases I will test individuals by themselves, so that I can focus on them entirely.

Recently we had a very large group of people testing for various levels of black belt. In order to avoid spending hours in the testing process, they had them do their kata in groups of three.