Remember the Milk makes it really easy to share individual tasks, but how can I make it automatically share certain groups of tasks (e.g., I'd like it to automatically share tasks tagged
#groceries with my girlfriend).
Though that would be awesome - You cannot share a Smart List. You can share a List , or numerous tasks but not a Smart List.
Here is how I would do it.
Search for your tag
And either select all your task you want to Share and just share that way (I think you already know this way) The other way, which I use it , is the List. So move your tasks tagged with grocery to a List called Grocery
Then sharing it with allow your friend to change items in your list.
There is a use case for this that worked for a couple.
source of use case : Tip & Tricks Forum morgan.parker