Are you asking how is the door certified to carry weight or are you asking about the procedures?
Although the ATR door is slightly narrower and steeper than the Dash-8 (you're correct, Bombardier, although de Havilland before that) it is structurally sound and has roughly the same weight bearing capacity - IIRC around 300 - 350 pounds or so - it has been a while.
The Dash-8 door does have a limitation that only one person may be on the door at one time. I actually had a Dash-8 door break on me when a rather larger gentleman got on the bottom step of the door while an equally large gentleman was on the top step. They didn't listen when the Flight Attendant said "one at a time please..." But when I say break, I mean that the two struts snapped in half. The actual door remained fixed to the aircraft even with all that weight on it and after an inspection by maintenance, we were cleared to go knowing we would be at a bridge at destination.
I would assume that the ATR door would feature the same one person limitation. Of course, the average passenger doesn't know or care and you'll have three people on the door at once. This does damage the door over time and could account for the shakiness you felt - although, it is unlikely that it would have actually been life threatening as it would take a lot for those struts and the door to separate from the aircraft - even though I have no doubt that's what it felt like!
Incidentally, both the Dash-8 and ATR are classed as Transport Category aircraft, not Commuter Category aircraft...a very common mistake, but one that sparks the ire of pilots from both types!