When I was younger and didn't own my own property, I paid pasture board and my guys stayed in a herd of 25 on 100+ acres. My horse was not herd sour at all and would come when called.
In fact, I remember one night at 10 pm - it was dark, but I was training for a ride and night riding was the only time I could ride sometimes, and I walked out to the pasture and called his name. I walked around and around in the dark (no moon, no flashlight) looking and calling and couldn't find him anywhere, just found all of the others. My dog was with me running around and helping me watch for skunks LOL!
I decided that maybe he just didn't want to come up so I turned around to go, and there he was right behind me! THE WHOLE ENTIRE TIME I was walking around calling him, he was right there. It was so dark, I couldn't see the tip of my nose. We went on a fast 5 mile ride and got back home and life went on...
I think that if they enjoy their human interaction, they will come - even if it means to leave the herd. Each horse is different!