I'm not 100% sure of that but I could see it as a potential problem if one or all the contributor were a fully exclusive contributor (at Istock) of if you were submitting images from an exclusive shoot (like at Stocksy). iStock could cancel your account "just cause", they don't always give a warning!
If no one is exclusive it will work but just get seperate model releases for every photographer from every model and ideally use different lenses and angles! One other option if you have many models is to take turns with the main group while the other photographer can do portraits with someone else on the side. Having many photographers leading the same group could get pretty messy!