There is a mailing list, libsigc-list@gnome.org for Libsigc++, to serve as a design discussion area as well as a place to report bugs. To subscribe, use the web page. Members can post directly, but all other traffic is moderated. There is an archive of messages, here.

Patches should be added to bugzilla bugs.

Open libsigc++ bugs
Submit a new libsigc++ bug here

libsigc++ is in the libsigcplusplus module in GNOME's git repository.