From the web site:
"The Music Encoding Initiative (MEI) is a community-driven effort to create a commonly-accepted, digital, symbolic representation of music notation documents. We strive to establish the design principles and the technological and representational requirements that will enable the discipline of musicology to take full advantage of digital technologies. We are working to provide guidelines and tools that can be widely used by libraries, museums, and individual scholars to encode musical scores for research, teaching, and preservation activities."
Tutorials are provided on the site: http://music-encoding.org/support/MEI1st
Info on MEI-L listserv is available: http://music-encoding.org/support/mailingList