Inspired by this infographic which I find confusing to use and makes assumptions that I think are no longer valid.
You should also check out these estimates based on age groups. This will be more specific for a group like a school camp where everyone is about the same age.

How many people in your area are reported to have covid (population prevalence)?

Reported Percentage currently infectious with Covid: (currrent community prevalence)

You may choose the current prevalence in the health district where most people attending the event are coming from:

Disengage Safety Protocols The maximum reported community prevalence of covid yet seen in NZ is 2.15%. Check this box to enter higher figures. Note that the reported community prevalence is adjusted by the % who report to obtain the true community prevalence

When is the event?

We assume case numbers currently halve every 315 days. We are working on a better calculation. If you don't like this estimate, choose Today's date and adjust the Reported Percentage currently infectious with Covid above to whatever you think it will be on the event date.

Event date:

How many people do you think report having covid?

The % who report: % or you think the true figure is times larger than reported

How many people do you think isolate for 5 days with covid?

Isolating percentage:
You might like to share this information. Just fill in the event description and the estimated number attending and then click Share/Copy. You can then paste the message into emails, Facebook, Twitter, etc

At the to be attended by about people

What's Changed (Last update: 4 Sep) | Who to blame? | @waynemcdougall | Privacy statement: No ads. No cookies. No tracking. No analytics. I don't collect any information about you. I store how many days ahead your event is, but it's not associated with you. I do this so I can work on focus efforts on improving forecasts. Your IP address is stored in log files which are deleted after 7 days. Those log files might get stored in backups which may be kept long-term. Worst case: Someone with a court order might be able to show someone using your network visited this site.

Caveat: The probabilities assume that people present are as likely to be infected as the average person in your community. If you associate with people that mask and test regularly and isolate when infected then you are much less at risk. If you hang out with antivaxxers and antimaskers and people who think covid is a hoax you a) aren't reading this and b) are at much greater risk. It turns out though: most people really are average in their behaviour.