The official site is http://www.musl-libc.org/ and this wiki gives further information maintained by the community.
To subscribe to the mailing list send a mail to musl-subscribe ΑT lists.openwall.com and reply to the confirmation message. (The mailing list is for development and user support discussions as well.)
The archive is browsable online at http://www.openwall.com/lists/musl/
The #musl channel on the Libera IRC network is where we discuss musl.