Manage group of users ?

Hi,
i need a way to handle group of users (not roles).
I have users and roles but, due to a new requirement , i have to group users and have, for example, a hierarchy like this:
ROLE : ExternalUser
GROUP  : Operator_CompanyA, Examiner_CompanyA, Operator_CompanyB, Examiner_CompanyB,...
Moreover, users must be able to see only record inserted from users of the same group.


I'm trying to add a Groups table (GroupId, GroupName) and a UsersInGroup table (UserId, GroupId) to those provided with membership and extend the Membership provider to handle groups.
Did someone have the same problem? Any suggest?
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