Has she been with another man? Usually, unless the circumstances are unusual, its a progression - one man, two men, many men. Usually, you need to help her build confidence, feel secure, protected, and safe to do a gang-bang.
Certainly there are exceptions, but we lack context to offer better advice. Sometimes sharing feedback about the comments on her photos can be a motivator. Have her imagine the possibility of hundreds of horny men, jerking, stroking and cumming, looking at her photos, imagining her pulling a train, or as the center in a gonzo, dog-pile style gang-bang.