It really does come down to what Bob says; you have to be more important than the outside draw of his attention. If his buddies are really important, then you have a lot to do.
Have you worked with him at liberty in a round pen? Has he ever really , honestly joined up wiith you? I say honestly, because a lot of horses kind of know the drill and they turn in and follow you around, but if you sent their feet off away from you, they'd leave you mentally. Or, if the buddies started calling, they'd leave. Honestly being with you means that his attention might go off there for a sec, but the smallest noise or movement on your part will draw it back.
the trick then is to DO something with it. most of us don't think far enough ahead in what we are doing wtih the hrose on the ground (or in the saddle , for that matter) to be able to really LEAD with committment.
So, before you ask him to move out, think about what you might do next.
Really good hrosemen dont' need to plan it out movement by movement. they can see waht the best next choice would be. But, for most of us everyday horse people, we need to be prepared with something, so the horse isn't just standing there while we twiddle our thumbs, and he leaves us there (mentally).
I do not say that you can't just have the horse stand , from time to time. And, some horses need MORE time to "soak" on the things you are doing, and if you don't give them time, they get confused and resentful.
But, I get the feeling that Chance is the kind that if you don't have the next moove up and ready to go, he'll take over the show pretty darn quick.