app/mailers/user_plane/invite_mailer.rb
def invite invite
      @sign_up_with_invite = User::SignUpWithInvite.new(invite: invite)

      mail to: @sign_up_with_invite.invite.recipient
    end
def invite invite
      @sign_up_with_invite = User::SignUpWithInvite.new(invite: invite)

      mail to: @sign_up_with_invite.invite.recipient
    end
Subject can be set in your I18n file at config/locales/en.yml
 with the following lookup:

   en.invite_mailer.invite.subject