title: Ruby on Rails: Devise: How to add "invite user to belong to model"?
body: <p>I have a <code>Group</code> model that has many members (<code>User</code> models). </p>
  
  <pre><code>class Group &lt; ActiveRecord::Base
      belongs_to :owner, :class_name =&gt; 'User'
      has_many :members, :through =&gt; :group_members, :class_name =&gt; 'User'
  end
  </code></pre>
  
  <p>The <code>User</code> model is using Devise. I need to add the ability for a User (Group Owner) to "invite" another User (who may or may not have a User record yet) to belong to the Group. How should I go about doing this? Has something like this already been built and packaged as a Gem?</p>