powershell to review users and add custom attribute??

I need to run this PowerShell command for several groups of users:

Set-Mailbox -Identity FOO -CustomAttribute1 FOO@BAR.org

I already set this up in the Details Template Editor.

where foo is the email identity and custom attribute is an additional, separate 'home company' email address separate from the domain mail.

I am looking for something like 

For each identity or samAccountName Active Directory group

Set-Mailbox -Identity FOO -CustomAttribute1 FOO@BAR.org

end for

each group has a different string for where I used BAR

Any suggestions would be helpful, then I can take them to a PowerShell forum.

Thank you, Tom

P.S. I went to all the work of adding a custom attribute in ADUC only to find out they can't be used in Exchange 2010/2013. ugh ugh UGH.