I'm not sure what you are after, but from the title of the post, the answer is yes. As long as the data is in the table.field, you can group on it. Basically, it has to be present, in an easily found way when CR reads the data.
I see you want to group by a customer, which is part of a larger group of customers, which doesn't seem to be to much of a problem, as long as there is some way to determine the customer...I am not sure about the reporting company, but I don't see why you couldn't display it on the report.
HTH