I would open the gate just enough to let them get in but not so much that others can get out but late enough in the evening so the flock won't want to wander to far from roost. Works best if the gate opens in of course.
Or
Wherever they are spending the night at probably isn't to far away. Watch where they go and once on the roost and asleep, unceremoniously snatch them and bring them backbone to the flock.
Or
Wherever they are spending the night at probably isn't to far away. Watch where they go and once on the roost and asleep, unceremoniously snatch them and bring them backbone to the flock.