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.
My husband and I use a shared RTM list for our grocery shopping. Each of us can put items on the list when we think of it from either work, home or via our smartphones and whoever is doing the shopping always has an up-to-date list. It works especially well as we often stop in to pick up one or two items, but we can also see what else needs to be bought. We use the 'priority' function to separate our items into the general areas of the supermarket, to make it easier to sort through a long list.
As an added bonus, if we are shopping together we can split up and tackle the shopping from different ends of the supermarket. We 'complete' each item on our phones as it goes in the trolley so we don't double up or miss anything. It probably looks a bit strange to other people when we stand in the vegetable section staring at our phones, but it really works well.
source of use case : Tip & Tricks Forum morgan.parker