I’ve been trying to figure out 
the best way of notifying all project members when a task is completed in specific projects. I will share my thoughts and the options considered.
But first: Why would I want to notify all Project Members of completed tasks?
Well, I believe that it is critical, in certain projects, that all project members know tasks are moving forward, not from a bird’s eye "Dashboard or chart "view, but actually getting notified: “Task 1 completed” “Task 2 completed”, etc. In fact in several projects I consider this notification more important than the Task Added notification
The first option considered: Add all project members as collaborators at once by multi-selecting all tasks. But if the project has, for example, 30 tasks, the project members will immediately get 30 Inbox “Added as collaborator” notifications at once - ( Inbox mayhem #1! 
). From that moment on, all project members would, of course, get notified of the 30 “Task completed” notifications during the duration of the project’s life, but also notified of every update/ comment to each task (Inbox mayhem #2!)
The second option and most typically adopted: Selectively add collaborators to specific tasks considering the relevance of each task for each project member to be notified when the task is completed. A brain-racking task in itself, and I would also forcibly add selected members to every future task update/comment.
The third option: Use an Integration to send an Email or a message to a Slack Channel every time a task is completed in a project. Unfortunately, this takes work out of the Asana ecosystem (I do my best to “stay in Asana”) and adds unwanted steps to a possible follow-up by the user notified (as you can’t easily preview all task info from outside Asana).In highly sub-task populated projects , this option, might be another notification mayhem (at lease with Zapier)…You would get completed notifications for all tasks and sub-tasks. Also, usually chat channels are populated by many users not in the project.
The fourth option: Don´t forcibly add anybody as a collaborator and let everyone make their own follow-up choices and live in their own My Tasks / Inbox “work bubble” and be happy with the idea that the sum of all “independent collaborative” efforts, in the end, gets the project completed.
Of course, anybody can go at any time to the project and check what’s been going on, but, in my experience it is uncommon for the majority of users to go into projects to do follow-up and check updates, most live in My Tasks / Inbox (except for us Asana Heroes / Champions / Gods / Geeks / Warriors who move and keep Asana alive in all organizations and check every detail over and over again, trying to maximize efficiency)
Perhaps a “Task Completed” option in the Project Member Notifications could be the solution? Project members with the option selected would get notified when tasks are completed without the need of being collaborators.
The case of Tasks Added vs. Task Completed Notifications
- What adds more value as notifications? Task Added or Task Completed?
- Perhaps Task Added notification is a “must-have”? and maybe Task Completed notification is a “could have”?
POST UPDATE: @Jason_Woods 's workaround is an interesting 5th Option:
