There are two ways to import targets from a CSV. You can either import directly in the Testing Wizard using a pop up or use the "Import" tab on the Add Targets page. Both options allow you to select whether or not the file has a header row, whether to update existing targets information, whether or not to deactivate any targets in the system that are not in the file being uploaded, and to suppress the verbose screen output to allow the upload to move more quickly. Deactivating targets will set their status to inactive so that they are no longer counted against your seat limit but leaves them in the system so that no data is lost.
Additionally, once you have clicked the "Review Import" button, the system will allow you to map the fields in the file to system fields and custom fields you have set up. In order to map custom fields, you will first have to create those fields in the group settings.
Once you click the "Import Now" button you will be prompted with the results of the import. You can return to the Manage Group Targets page by clicking the "Manage Group" button.
To see how to properly format a CSV file for import see this article How to properly format a CSV file.