With iCloud Plus you're able to add custom email domains (really nice and tight integration with Cloudflare DNS for all the MX records) -- was a breeze to set up. Haven't put it through it's paces yet, though.
Weirdly, you can only have three email addresses per domain. So you can't really use it for a typical family domain, even though it supports the family group (which can be six people).
The other users in your family group will be able to add 3 addresses of their own; the 3 you see are just the email address for your root@yourdomain.com account. User1 will have their own set of 3; login to iCloud.com from one of the family accounts and you'll see.
