You can be notified of a new participant joining a TeamRaiser by setting notification at the participant level:
  1. Select edit on the TeamRaiser.
  2. Select to "Manage Participation types"
  3. Select edit on each of the participation types that you would like to receive notification for.
  4. Select step b. Select Type Options.  Once here add the email addresses of anyone that should be notified of a new participant.
  5. Save.

Moving forward you will be notified of new participants signing up for all types that were customized.