Add non-Primary email address to Group
In CiviCRM you cannot assign email-adresses to groups but only contacts, including all their related information such as email-adresses.

Could you specify, what you want to do with the contacts in question (e.g. sending an email to a specific address, create a pdf-letter...)? Maybe there is another solution for your task at hand.

I think the non-primary email-addresses are not found by simple or advanced search due to performance reasons - there is an older threat here:

http://forum.civicrm.org/index.php/topic,16221.msg115088/topicseen.html#msg115088