Does anyone on here know anything about this specialist Broadway musicals forum, which apparently includes official/original instrumentals and audio stems from numerous Broadway shows and cast albums? This incredibly intriguing content is not viewable to visitors unfortunately and registration has been disabled also. Many thanks in advance.

I don't, no. But if registration is disabled, that often means they are not admitting new members. Or, they are invite only now. In that case, you'd have to hope someone around here is a member, and can recommend you.