If one person is unreliable, why would a group of people make it any better.

You misunderstand the übermensch. It's not a kind of person, not some state you can achieve. Rather, it's the future potential of humanity, Nietzsche's suggestion for a new guiding star to give purpose and meaning. You can also view it as an inversion of the christian god; god is our father, created us, lives above us, while we create the übermensch through our actions. God exists in some seperate dimension/layer and christianity tells you to look up, away from the material world, while the übermensch, as the result of your and everyone elses actions, focuses you on your actual, physical life.

    The opposite of the overman [Übermensch] is the last man: I created him at the same time with that. Everything superhuman appears to man as illness and madness. You have to be a sea to absorb a dirty stream without getting dirty.