APP_HOME_URL is needed, as found here: https://community.getgrist.com/t/networkerror-when-attempting-to-fetch-resource/1653/4
This creates a functioning anonymous instance with no login auth defined. Unclear of how users are managed, maybe all in cookies, does weird stuff like giving user named like @guest and @you (possibly to multiple users simultaneously). Multiple people can edit the same table, data will update to other user, but there's no notification of collaborative editing: no user list, and no cursor to show what other user is doing.