How do I upload a list of email addresses that need to be changed to 'email opt-out' in the Contacts? (Group Edition of SF)

What you could do is this:

• Back in
  Dataloader.IO against the Email Field on the CSV, map the same to Contact ID
• Under the same, select Lookup via : Contact Email

This in turn finds the Contact with the same Email and fetches it's ID and replaces it there while inserting the same to the Database.