Cannot properly push object to another object

I'm doing Asabeneh 30 day JavaScript and an exercises requires us to make a sign up function which passes an object newUser to an existing object Users. I can't seem to properly push my newUser object properties to object users. This is my approach to the problem. I've been recently working with object methods.

const users = [
  {
      _id: 'ab12ex',
      username: 'Alex',
      email: 'alex@alex.com',
      password: '123123',
      createdAt:'08/01/2020 9:00 AM',
      isLoggedIn: false
  },
  {
      _id: 'fg12cy',
      username: 'Asab',
      email: 'asab@asab.com',
      password: '123456',
      createdAt:'08/01/2020 9:30 AM',
      isLoggedIn: true
  },
  {
      _id: 'zwf8md',
      username: 'Brook',
      email: 'brook@brook.com',
      password: '123111',
      createdAt:'08/01/2020 9:45 AM',
      isLoggedIn: true
  },
  {
      _id: 'eefamr',
      username: 'Martha',
      email: 'martha@martha.com',
      password: '123222',
      createdAt:'08/01/2020 9:50 AM',
      isLoggedIn: false
  },
  {
      _id: 'ghderc',
      username: 'Thomas',
      email: 'thomas@thomas.com',
      password: '123333',
      createdAt:'08/01/2020 10:00 AM',
      isLoggedIn: false
  }
  ];


const newUser = {
      _id: 'fj3093',
      username: 'str8cho',
      email: 'str8cho@str8cho.com',
      password: 'kladjflakjf',
      createdAt:'08/01/2020 9:00 AM',
      isLoggedIn: false
  }

const signUp = function() {
  let newList = Object.assign({}, users, newUser);
  return newList
}

console.log(signUp())