We maintain two mailing lists for the Twelf project through mailman. The moderated mailing list which is used for announcments related to Twelf is called twelf-list. The other is called twelf-developer. It serves as discussion forum for people who are working on the Twelf implementation.
If you want to join twelf mailing list visit http://www.itu.dk/mailman/listinfo/twelf-list.
If you want to join twelf-developer mailing list visit http://www.itu.dk/mailman/listinfo/twelf-developer.