Files permissions with organic groups
The submodule 'Organic groups field access' of the OG project should be able to help you manage file access based on Organic Group permissions.

You can create private Organic Groups by enabling the OG submodule 'Organic groups access control'. 

Depending on the useless: Make sure to add a 'Group visibility' field to the Group bundle or a Group Content visibility to the group content bundle at admin/config/group/fields.