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Is there some FOSS (or even proprietary!) software that does this?

I want to communicate to people who don't like to create accounts, they're like this:
* people want to hear about events (improv, Zumba, etc)
* they WILL sign up to a mailing list
* but they will NOT "create an account"

For an example, Substack does this for newsletters! You can "sign up to a newsletter" before creating an account (but Substack is not events-focused)