Reviving a 1½ year old thread :P
Mine are very close, and do not like when one is out of sight, and individuals are hesitant to leave the others.
My suggestion is similar to Ali's.I'm taking advantage of the spring grass (of which there is non in their pasture). I take one horse out to grass, and all we do is really just walk and eat. Not the most exciting acivity (for me), but my priority is just to make it a pleasant experience for at least the horse I have out with me.
Whomever is left in the pasture call out for the one horse I bring with me, and in the beginning, the single horse would call back and be wary of walking too far or getting out of sight. I'm seeing a change in behaviour and willingness already though, having hardly done anything at all but just let them go further at their own pace. I may suggest going towards a grassier area, as to let the horse know I know stuff about nice chow that they don't
My hope is that I'll eventually be able to actually go for a proper walk with a single horse at a time. I feel that hope might actually become a reality this year! (lol)