I haven't read any of the replies however there are several methods. Every horse (and trainer) works differently so I like to keep a few tricks up my sleeve.
My first reaction is always: if she walks/trots/runs off you chase her. Be loud, try to use your body language to keep her separate from buddies so your not making a sport of chasing a herd of horses - because it wont work. You can typically out smart them and you jog around keeping the horse going with out having to RUN after a horse for 20 minutes. She'll eventually give in and lower her head/lick her lips/ask to stop/stop/come towards you etc.
You can move her to a smaller pen until she is trust-able at being caught.
If this doesn't work for whatever reason switch up her game. When she goes to walk off you turn and walk away. When she stops, come towards her, when she walks off you turn and walk away. Continue until you've got a horse who then wonders WHAT you are doing and they'll usually end up following you around because rather than chasing them your switching up the game.
There are many ways to train a horse to accept being caught, what ever you do - do not use bribery (no treats, grass, grain buckets) As you'll then become that parent who give the crying child a cookie or says "When I count to three you'd better come here or I'll...." but nothing happens when you get to three and they know it ;)