how can a collaborator know when I have updated a task or comment
Here's are steps to ensure that they'll get the notifications:
1. Make sure that you have the notification for comments enabled
2. When adding comments, make sure that the user is one of the selected collaborators that gets notified about the comment. You can modify this at the bottom of each notes window. Click "Change" and you should be able to select the users that you want to notify.