A coworker and I are collaborating on a system and we keep bothering each other with code updates. It'd be very helpful to subscribe another member to a task that I'm working on so they know what I'm doing. Yes, I can just tell them verbally, but when it's 1-2x/day...it'd be nice to have that right in Trello. I don't want to add them to the task though, because only I will work on it.
You can add the other user as a 'member' of the board yourself, and then they will get all updates on the board.
if by 'task' you meant an item in a check list, you can @mention their user name in the task item text and they will get a notification (same with @mention a user in the description)
Have them Subscribe to the card, this is the best way to be notified of card actions without being a member o the card. They will need access to the board in order to do this.
To subscribe: Open the card and find the Subscribe button on the right side.