As far as I know, it is not required for the external member to be from an NGO alone. The requirement is that the person has familiarity in dealing with such cases both internally and if need be , externally. ( For cases where one decides to file an external case). Also provide sound unbiased standpoint.
Hence most Org I have worked with, gets someone from relevant NGO or Law Firms or ex advocates, etc as they would have dealt with the complexities of multiple clients. @Rohan Mankad