class CreateUsers < ActiveRecord::Migration
  def change
  	create_table :users do |t|
  		t.string		:first_name
  		t.string		:last_name
  		t.string		:user_name, null: false
  		t.string		:email, null: false
  		t.string		:password_hash
  		t.boolean		:in_office

  		t.timestamps
  	end
  end
end

class CreateUsers < ActiveRecord::Migration
  def change
  	create_table :users do |t|
  		t.string		:first_name
  		t.string		:last_name
  		t.string		:user_name, null: false
  		t.string		:email, null: false
  		t.string		:password_hash
  		t.boolean		:in_office

      t.references  :user_group

  		t.timestamps
  	end
  end
end

Add reference to user_group in User migration
Add reference to user_group in User migration
