There's no per-seat fee, so invite anyone involved in the hiring process.
It is not necessary to share the spreadsheet with interviewers for their addition in Settings sheet.
If you add email as a note in the cell containing a recruiter's name, that is used for showing personalized filter for that recruiter.
If you add email as a note in the cell containing an interviewer's name, that is used for adding that interviewer in draft Google Calendar invite.