Thank you for your help.
I am not sure why they want it, I am an admin, and I was asked by the teachers several times that they need excel file with names/ID of the students who submit a particular assignment.
course Administration>Reports>Course Participation>select the activity, role, action and the group
you will have list of students who viewed/post a certain activity.
and you can send messages to the selected users.
An obvious way to do this is "completion tracking", which does need to be enabled at site level and then at course level. You then set the criteria for each activity you want tracked (eg work submitted) and you can view it in a spreadsheet like report that is downloadable.