concern.rb

class User
  belongs_to :group
  validates_presence_of :first_name
  class << self
    def find
      "foo"
    end
  end
  def awesome?
    false
  end
end

module.rb

module UserConcern
  def self.includes(user)
    user.extend(ClassMethods)
    user.class_eval do
      belongs_to :group
      validates_presence_of :first_name
    end
  end
  module ClassMethods
    def find
      "foo"
    end
  end
  def awesome?
    false
  end
end