Currently you can add multiple reader groups when you create readers individually. However, if you import, you can only assign one. Therefore, once the readers are imported with the one reader group, you have to go into each reader individually to add the other group(s). We have a situation where there are categories that are public to all readers, and then each client has access to their client specific category. All of which are controlled through a 'public' reader group and then 'client specific' reader groups. So minimally every client gets two reader groups.