Many thanks for your answer Janis! Suggestion number 2 is what I was looking for and it works very well. You can see the result and what I am trying to achieve in another topic I have created: Expand a specific row or column
The next question I have is how, instead of Assignee, I can calculate based on users from a group in a custom User Picker field. I have opened a separate ticket for this: Make calculations based on users from a group in a custom User Picker field