CRM-2839 I tested on Demo by uploading 7 contacts for whom I only had an email to assign to an existing group - three of them existed with first+last name in dbase - four were new - with no first or last - it confirmed it 7 rows were imported but
"7 contacts added to this existing group. 4 contacts NOT added (already in this group)."
I tested and all 7 were now added to group.
I retested with another 7 of which two I had pre-created - and got same type of message saying 5 contacts NOT added.
My interpretation is that it is reporting 'contact not added' for the ones for which there is no first or last on my import, or in the dbase.
Lobo's comment was "(bad counts / bad message)" if that explains it better.