While not all horses are super human focused, horses in general are social creatures. Show your horse that spending time with you is awesome. Feed, treats, grooming, interesting tasks, and lots of praise usually will do the trick. Make certain that your horse has enough paddock time on its own that being out with its buddies is the norm and nothing to be excited about. Be consistent with your ‘come here’ signal, whether a whistle or a yell, and soon enough, your horse will want to be with you.