Invite.rb

1.
----

InvitesController

def new
 - show invite form
end

def create
 - create new record with email and md5 of email + any additional info
 - send email with registration link that includes md5 of email 
end

2.
----
User clicks link to Devise sign_up page:

mydomain.com/users/sign_up?invite=<md5>

Devise RegistrationsController is extended:

class Users::RegistrationsController < Devise::RegistrationsController

def new
 - Check for valid md5 and retrieve record
 - Prefil user object with info from invite object
end

def create
 - Check for valid md5 and retrieve record
 - Save user
 - Do anything else that needs to be done with any additional info (create permissions, etc.)
end