a horse that does this isn’t herded up with you. you should be their source of comfort and support. when i have been working consistently with my mare, she ignores the calls of other horses and doesn’t call out herself. if i leave her without work for a week or more, she goes back to seeking comfort from other horses.
to get your horse to find comfort in you, make her work around other horses, and let her rest when she’s alone with you, or chooses to leave the other horses.
if you are out riding alone and she calls to other horses, change the subject. serpentines, circles, and especially ask her to back up. horses can’t focus on two things at once.
but most of all, build a bond with your horse that is so strong that she isn’t interested in other horses when she’s with you.