To collaborate at the task level, you can add subscribers to tasks so that they receive notifications on updates or comments.

When you first add a team member to a task, they will gain access to that task if they didn't have access before, and will receive a notification telling them that you added them as a subscriber of that particular task.

If they stay subscribed to that task, they will then receive notifications for new task comments and when the task is complete.
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You can add subscribers to a task or they can subscribe by themselves, so they stay informed and can participate:

Alternatively, you can @mention team members in a task comment, which will subscribe them to the task automatically. Learn more on @mentions.