Alleged authorities should only be used when the authority is reporting on his or her field of expertise, the authority is reporting on facts about which there is some agreement in his or her field, and you have reason to believe he or she can be trusted.
Alleged authorities can be individuals or groups.
The attempt to appeal to the majority or the masses is a form of irrelevant authority. The attempt to appeal to an elite or select group is a form of irrelevant authority. Brad Pitt was seen wearing Designer Bob's sunglasses, so they must be the best sunglasses to wear. Nobody is a better judge than the public opinion. Pacifism is a good idea because the brilliant scientist Einstein advocated it.