Authorisation Group for Policy Dcouments

I am creating new document type  for Policy documents. There are multiple business units in organisation. Each Business unit will have there own policies. For example BU1 and BU2 create DOC1 and DOC2 of type POL. User from BU2 should have read only access to DOC1 and should have edit access to DOC2 similarly User from BU1 should have read only access to DOC2 and should have edit access to DOC1.

I am planning to use authorisation group in this scenario. Can any one please help me with detailed procedure for creating authorisation group.

Thanks and regards,

Salil Joshi