Using the ‘When value is changed’ event of the dropdown.

In the solution below, I have a single dropdown that will be used for the entire repeating group. You select the users on whom you want to perform this action and then choose the option from the dropdown.

Once you do that, you will see an alert on the screen informing you that action X will be executed on Y users (just so you can see that the event has been captured). In a real scenario, instead of showing the alert, you would execute the desired action.

Don’t get caught up in the UI/UX details of the page. It was created quickly just to give you an idea of how it can be done.