Access rights on Accounts based on "Customer group" field?

Dear SCN Community,

we have a customer with 20,000+ accounts in their system and some sales people responsible for them. Our customer does not want his sales people to see all of these accounts. To be specific, they only want to let their sales people see accounts they are responsible for AND are part of a specific customer group.

The responsibilities are derived from the postcode of an account and automatically set. Therefore it is not possible to change the responsible employee.

Is there a way to define access rights for accounts based on the "Customer group" field?

Thanks for your help and best regards,

Georg