The "Add-DistributionGroupMember" powershell cmdlet is available in Exchange 2010. To do this ensure you can access the CSV file on one of your Exchange 2010 servers and run it on that server. On a normal client desktop "Add-DistributionGroupMember" is not available.

Check the CSV File Output:

Import-Csv .\StaffForDistGroup.csv

displayname

---------

John Doe

Bob Smith

Andy Barnes

Jane Smith

Tom Jones

Harry Best

Ben Dover

You can have various fields in the CSV file, its just important somewhere there is a displayname/samid/DN etc so it can be used to add to the group. If you dont have this like I have in the past had first name and last name fields you can just concatenate these to get the full name (which might be your display name?!).

Add the Users to the Distribution Group based on the "displayname"  field:

Import-Csv .\StaffForDistGroup.csv | ForEach {Add-DistributionGroupMember -Identity "All Staff Payroll" -Member $_.displayname}

And there you have it, all the users will now have been added to the Distribution Group.