hmm... I think you need to think about why they may be choosing to roost outside. What's so appealing about the outside roost? If you don't want them to roost there, can you eliminate that roost?
Does your coop have a window? You mentioned that the door is a new feature, when you close the door is the coop absolutely dark now when it might not have been before? If the new door means no light from the outside, you may need to add a window. If the new door means no air flow, you may need to add a vent.
You also mention a new chicken. How does everyone get along? Is there enough roost space that they don't have to all cram together if they don't like each other? When I added 2 new girls the 2 new ones would roost together in one spot and the old chickens would roost in a different spot, they all got along, but they would not roost together for months. They had enough room to be able to do this in the coop, but if not, I don't know, maybe they would have resorted to roosting outside or sleeping in the nest box.
Just some things to think about.