First and foremost you can invite all the customers (who need visibility into the project) to the project! That way they’ll get weekly automated project status emails. That is usually sufficient for most customers.

The second thing you can try (if more granularity is needed) is to add those customers as watchers. Any task they are set up as a watcher, they’ll receive any email regarding that task (when attachments are uploaded, when it runs late, when it’s completed, etc)

Hope this helps! Let me know if it doesn’t!