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a leader
two people singing together is a duet
duet
A person who watches movies might be referred to as a viewer. Also I have heard people call them a movie buff, which means they know a lot of stuff about movies.
Call their agent and add the other person to their policy.
a person who studies people but not an psychologist
A person who wanders around with no home is commonly referred to as a nomad or a vagabond.
Quartet
Choir
A duet.
A nice person.
A person who laughs at others is typically referred to as a bully or a mocker. Their behavior can be hurtful and can negatively impact the well-being of those they are mocking.