input: Write a python function to add a new user to the database and check if a user already exists.
  assert user_exists('existing_user') == True
  assert user_exists('new_user') == False
output: users_db = set()
  
  def add_user(username):
      if username in users_db:
          return "User already exists."
      users_db.add(username)
      return "User added successfully."
  
  def user_exists(username):
      return username in users_db