First, run this command on a domain controller to extract the members of a security group:
net group "Security group name" >c:\groupmembership.csv
Then run this in an Exchange 2010 shell to extract the mailbox names from the database:
Get-Recipient -PropertySet ConsoleLargeSet -ResultSize '9000' -SortBy DisplayName -RecipientType 'UserMailbox' -Filter '((Database -eq ''CN=DATABASENAME,CN=Databases,CN=Exchange Administrative Group (YOURADMINGROUP),CN=Administrative Groups,))' | Export-csv C:\userslist.csv
Then paste your names lists into excel in two columns, one named group membership, and one database users. Use the below formula in the third column to find the names that occur in both columns of data.
=VLOOKUP([first column entry,[Range E.G. A:A],1,FALSE)