hi
How i would approach this would be to query the list and get all the assigned users. Then i would perform a remove duplicates collection operation to get a deduplicated list.
Then i would do a for each on each member of the collection, query the list for all items assigned to the relevant user and the build the email body from the return collection of items, using foreach again to iterate through the items and get the desired output.
should help with the operations.
Let me know if you need any more help with the workflow
There's a similar question here: Multiple List Items Sent to One Person that had some great input from Giacomo Gelosi, including his blogpost on the topic