Timecounts tells one person from another in your database by using their email address. Email is in the unique identifier for your contacts, so all imported spreadsheets must contain an email field. Only people with an email address will be imported or updated.
If you have someone in your list who does not have an email address (or shares an email with someone else), you can create a placeholder email address to identify this person using the email domain @noemail.timecounts.org. For example, firstname.lastname@example.org.
Tip: make sure to create a unique placeholder email address, or you'll end up with duplicates!