I don't think we've ever charged a friend group or other non-incorporated group of people a dime for self-hosted notifications.

For the community tier, you don't have to do anything up to 10 users.

If your server has more than 10 users, you fill out a brief form (https://github.com/zulip/zulip/blob/main/templates/corporate...). We work hard to consistently process these requests within a couple business days, and the vast majority of communities are approved for full sponsorship without further interaction.

(Large communities managed by a business are quoted nonzero but extremely discounted pricing for self-hosted notifications).