If registrants are assigned into different groups (refer to Group Assignment), to email some specific registrants in a particular group, you can run Custom report and group them by Group assignment, then send out email. 

Below are steps on how to email registrants of a specific group. 
  1. Click Reports under desired Season
  2. Click New custom report button
  3. Click Customize button > select Group records
  4. Next to Group by, select Participant: Group assignment
  5. Select desired Group assignment
  6. Click Group records button
  7. Select checkbox next to desired Group
  8. Click Email selected people button