How to add a user to a specific group while subscribing to a list

Question

I am using gibbon gem

I am running this on console and its adding user in my subscribed_list. Works like charm.

gb.lists.subscribe({:id => list_id, :email => {:email => 'mohit.jain@example.com'}, :merge_vars => {:FNAME => 'Mohit', :LNAME => 'Jain', }, :double_optin => false})

Now I want to subscriber to groups: Like I have a group " User Type" which has some predefined 4 values. How can I map this User Type in subscribe method?