Fantastic questions! You start with a start of session signal and added to this is your observation of which animal doesnt let the other ones come in. This individual you reinforce for the others to come. Slowly you work your way to start of session signal means everybody comes to the same place if thats your criteria. From there you can start working separations, stations etc.
Start by coming closer to them and reinforce for attention to you 🙂
You can also decide to focus on one animal at a time till you have all animals on a place you want them to be on one specific signal.
If the animal in this case the horse reaches criteria and you are like hmm.. maybe the rest didn’t come because this one chased them first… then criteria is not met. You ask a group to come not an individual. 🙂 Does that make sense?