I was having a problem with my chickens (now two months old) returning to the coop after free-ranging all day. They decided to roost in the trees outside the coop rather than go inside. They had access to the food that was inside the coop all day. I decided to remove the food when I let them out and return it to the coop in the late afternoon and they all went inside for food. Success. Or is it? This means they have to forage for 10 hours without grains all day. Today it is pouring rain and I let them out but left the food in the coop believing they wouldn't be able to forage in the rain. We shall see if they return to the coop tonight. Hopefully, they will. Chasing chickens in the semi-dark is no fun.
Like jdleppo said above, chickens are easy to train. However, they're also creatures of habit. Their habit now is to roost where they've been roosting, outside. Since you want them inside, you're going to have to change that habit.
New adults to our flock just follow the others into the coop, but young birds sometimes need a few days of education to learn where to go at night. At dusk, if they look confused about where to go, we'll bend over, gently pick them up, and slowly walk them (holding them just above the ground while bent over) to the coop ramp. Again, we move slowly; this is so they can see the path we're showing them. At the ramp, with them still just off the ground, we move them up the ramp and set them at the entrance. After no more than 3 or 4 nights of this, they seem to get it.
If your chickens have always roosted outside and they're adults, this may take a little more training. Then there's always one or two that still don't get it after living in the coop for months. Maybe they just prefer to be outside. One of ours wants to be on the opposite side of our split coop (where our juveniles live), and she'll do everything she can to get in that side. About once per week, she'll have worked on getting in that other side long enough that she can't see to find the door to her run and her side of the coop. It happens...
For what it's worth, chickens don't have coops in the wild; they naturally roost in trees to avoid ground predators... Are these chickens new to you? Were the roosting outside with a previous owner??
