The following SQL Script provides an output of all AD Groups that have access to users' mailboxes
USE ArchiveManager
SELECT g.GroupID,
g.GroupType,
g.Name AS GroupName,
m.MailBoxID,
m.Name AS MailBoxName
FROM GroupMailBox AS Gm with (NOLOCK)
JOIN MailBox AS m WITH (NOLOCK) ON gm.MailBoxID = m.MailBoxID
JOIN [group] AS g WITH (NOLOCK) ON gm.GroupID = g.GroupID
ORDER BY GroupName