How old are they? If they are still little and need heat then I would leave them in there. If they are older, then open the door and make sure you have a bit of an enclosed run. They will go in and out when they want. They will all go inside at dusk if they know that is their home. Once you know they are ok going in and out at dusk, you can open the run and they will free range... and usually come in at dusk
I don't give my chickens a lot of treats. I only give them out when I call them (I whistle actually). They associate the whistle with something unusually great and come running. If I need my girls to get in the coop, all it takes is a bit of a call and a handful of leftover veggies or rice!