Why is groupname field blank in radacct

I installed freeradius and have noticed that all while other fields are filled in on the radacct table some are not. Of particular importance to me is the groupname field. I need this field because I need to know which group the customer is from so that I charge them accordingly. A person may belong to more than one group and we need a particular group the person used to log into the system. How can we cause the groupname to be written to the radacct table? Thank you.