After CRJ700 main cabin door retraction system fails and is MEL'ed; Flight Attendant is not briefed by Captain on procedures to follow when opening and closing the door. Door repeatedly falls open causing damage.

Narrative

The main cabin door retraction system failed to close the door. The door fell back onto the ramp. The Flight Attendant attempted another closing with no success. Maintenance was called. The door was put on MEL. The Ramp crew manually then closed the door; but the Flight Attendant didn't catch and pull it in and it again fell to the ramp. She was briefed on the catching; etc. The next time was successful. The flight departed. At the ramp on arrival; Ground agents were standing by to open the door. I signaled via hand I was shutting down and to be ready. Normal shutdown was performed; the door opened; and again fell to the ground. No obvious damage was detected on postflight. The ground crews mentioned they were surprised by the door opening by the Flight Attendant. I was made aware that damage had occurred following the trip. I obviously didn't ensure the Flight Attendant understood the events that were to follow. I would like to believe otherwise; but I can come to no other conclusion. It is the duty of the pilot in command -- the Flight Attendant is not trained in things mechanical and as these events are encountered that responsibility is the Captain's and no other.