Just because I stumbled upon this topic and the documentation is missing, at least from here…
How to assign multiple email addresses to a group
You can add multiple email addresses to a single group by separating them with a |.
Browse to https://discourse.example/g/group-name/manage/interaction, and then in the Email field you can enter:
group-name@discourse.example|info@discourse.example
Then your group will be able to receive email from both group-name@discourse.example and info@discourse.example.