I always keep a mason jar filled about 1/3 full of scratch (or you could use BOSS) on the coop shelf. When they hear me shaking that jar, with the sound of the scratch hitting the lid and I'm yelling "here chick, chick, chick", they know they're going to get a treat. Works great to call the flock in from free ranging if I need to get them in because of a T-storm or I need to leave the property. Also works great when I'm away and someone else is watching them for me. I know that all the sitter will have to do if someone escapes the coop is shake that jar and she will have everyone's attention. To train them just pick a time when they're outside, shake the jar, yell your words of choice and then drop a bit of the treat. Repeat daily until they've got it, which should be pretty quick, they are highly motivated by treats, lol.