invitations_controller.rb

def redeem
  # Lookup invitation, 404 if not found
  # Build new user.
  # If user is valid
    # If existing subscription, add user to subscription
    # If adding to plan, create new subscription w/ as group leader
  
  # If successful
    # Create invitation redeemed event
    # Delete invitation
    # Send welcome email
    # Send invitation redeemed email
    # Log user in
  # Else
    # re-render form
end