Well, there are 3 parts to this:
Your registrations, your attendance, your retention.
Registrations:
Start acquisition 2 weeks before the event. An ideal approach would be a 1:50 ratio. For every 50 people you invite 1 would register. So if you want 300 registrations you need to invite 15000 people. Ideally 1 message every 3 days for 2 weeks. 1 message everyday in the last 3 days. Try different copy variations to see what works best. Sometimes just sending a calendar invite also works. Gray hack, use it only when you are low on numbers.
Attendance:
Start reminders 3 days before the event. Start by blocking calendars and sending confirmations. Then send out an update email where you share some more information. One night before the event send out whatsapp messages with all the details as a reminder. 2 hours before the event also send a whatsapp message saying we cant wait to meet you virtually.
This will give you a 50% attendance rate.
Retention:
The quality of your content, gamification, and ongoing engagement will decide if people stick through the end and will they come back the next time you invite them.
Post event send a thank you note seeking feedback. Then the next time you invite them tell them that you worked on their feedback. Theyll be more than happy to rejoin.