If she's severely emaciated (all the bones visible, no fat on tail etc) immediate vet care is probably necessary, if she's just thin (pelvic girdly not clearly visible, thigh bones not clearly defined, some fat on the tail etc), you can try the following first.
Immediately remove her from the group and place her in a cage lined with something like paper towels or repti-carpet that can't be eaten. Offer your leo food once she's settled in. Sometimes the leo at the bottom of the heirarchy will get so bullied that they kind of stop trying to compete for food etc. If this is the case, she should start eating and recover quickly if isolated.
If she doesn't eat, try Marcia's slurry (
http://www.faunaclassifieds.com/foru...ghlight=slurry ). If after you give her the slurry and she produces a sandfilled stool and starts accepting normal food, her problems were likely due to impaction and as long as she's passing the sand she should be able to clear it on her own.
If she eats and produces loose runny stools, or if she eats, but fails to gain weight she may have a parasite or bacterial infection that requires treatment (take a fresh stool with you to the vet). If she doesn't produce a stool or produces very little or nothing but urates, she's probably got an impaction that she can't clear and needs medical intervention.
Some people have success with sand, but for every success story I've seen 10 posts about impacted geckos. Personally, if I were to use sand I would use real sand before any type of calci-sand. Here's a page with some info on calci-sands
http://www.pythons.com/calcium.html .
-Alice