Presume this question is related to contractors forming a private limited company as a kind of alliance and not a PLC (a PLC, public limited company, can be formed but the share capital alone is £50k and is a complicated process with lots of governance).
If contractors have joined together to form a company they could still be exposed to IR35 whether by definition of their company being a personal services company or if the work being done is as a disguised employee of the end client. This article provides much more detail.
Speak with an IR35 expert