Followups and reminders can be different kinds for your hackathon. These all depend on the registration, pre registration and other significant events within your hackathon. Please keep in mind not to spam your attendees because that affects your hackathons' experience significantly.
The first followup we recommend is for the hackers who have done the pre registration for your hackathon, informing them to register for the hackathon after the main registrations are up. We recommend sending out just 1 or a maximum of 2 followups, which should be also cross referenced from your current registered hackers' list and sent to those who have not registered.
The other followup is for the last years' attendees informing them about the new iteration of your hackathon and asking them to register for it. The followups here should be similar to what you do for the pre registered hackers, first one done while rolling out the registrations and the second one during the last month to the hackathon.
For the registered participants, the followup looks a bit different. To keep them excited we recommend informing them about the major happenings within your hackathon, but again that email should also be confined to a maximum of 1 where you can also ask them to stay in touch by joining the social handles of the event.
Finally, a week before your hackathon, it is the perfect time to remind everyone involved about what needs to happen at the event. Be sure to reach out to the different people involved according to their role at the event:
Attendees: Send clear instructions about when and where to check-in.
Sponsors: Sponsors also need clear instructions about when and where to check-in.
Volunteers and Fellow Organizers: Confirm all volunteer sign-up and designated duties. Send clear instructions about when and where to report.
Judges and Speakers: Check-in times and overview of what they should expect day-of.