Can I cancel just one passenger on a group flight booking?

Often yes, but it depends on the airline and fare:

• Check Manage Booking for the option to cancel one passenger.
• If it isn’t available online, contact the airline or agency, because group changes often need an agent.
• Ask what happens to the others. Some fares reprice the remaining tickets.
• Ask about refund or credit for the cancelled passenger in writing.
  Name changes usually aren’t allowed, so you can’t just swap in another person.

Get the new total confirmed before agreeing. Here is a simple guide on how to cancel a flight booking that covers the basics.