The scenario is: you already have the contacts in your account and you need to re-import the contacts from an existing .csv file in order to put them into a discreet group for marketing purposes.
During the import you match the fields across (inc. email), use email address as the unique identifier in the duplicate processing section, then in the groups section you add to either an existing or a new group.
BUT that all goes smoothly until you check the group and the number you were expecting is far less...
A: Check your Archived contacts and the ones you see there you can tick their box and then click on the button below "Restore selected". This will restore the contacts back into the "Live" list and then you will be able to see them listed in the group.