Call them, and provide some kind of a treat.
My favorite "treat" for this is a dish of wet chicken food: they seem to really like it, they cannot pick up bites and run off with them, and there is no need to worry about them eating "too much" and skipping their regular food (since it IS their regular food, with water added.)
You can also use scratch or mealworms or table scraps or anything else your chickens like. It works best if you only use small amounts of the treat, so they never get quite as much as they want. That encourages them to come running every time you call and offer the treat.
For some kinds of treats (like bread crusts), the chickens will tend to pick up pieces and run around chasing each other-- not helpful, when you want them to go into the pen and stay there!