Once an Interview is created the recruiters can ask the participants to join the interview session by a custom URL or an email invitation.
When a quick pad Interview is created it generates a custom URL for that session. You can copy that URL and share it with the participants in their preferred mode. HackerRank does not send any notifications for quick pads. The recruiter has the responsibility to provide the generated links to all the participants.
When an Interview is scheduled it prompts a message to send an email invitation to all the participants. When you click on Send HackerRank sends out email notifications to the candidates as well as the interviewers specifying the details such as the date, time, and a link to join the interview.