Devise default mailer won't work for you, but it's quite easy to set up a custom mailer and define there your own subject_for helper:
# in config/initializers/devise.rb:
...
config.mailer = "CustomDeviseMailer"
...
# in app/mailers/custom_devise_mailer.rb:
class CustomDeviseMailer < Devise::Mailer
protected
def subject_for(key)
return super unless key.to_s == 'invitation_instructions'
I18n.t('devise.mailer.invitation_instructions.subject',
:invited_by => resource.invited_by.try(:full_name) || 'Someone')
end
end