The Global Challenge can only accept teams of seven. If someone pulls out, we recommend that the person is replaced by the Team Captain via the ‘Manage teams’ page so the team isn’t left behind. The new team member will inherit the previous member's entered steps prior to the change.
If you don't have seven people yet, speak with more of your colleagues and encourage them to get involved. Don't forget; it's also possible to form teams of colleagues from different offices and countries as well.