The LJ-Community is not a mailing list - so we've got this as well, as an extra layer of communication with database, photos, polls, calendar and more. Feel free (but not obliged) to join there as well.
If, however, for any reason you'd prefer not to have or get a Yahoo-ID at all, you can of course still join the mailing list that Y!Group provides, with no strings attached. For the moment you won't be missing much, as all the extras on top of the mailing list have yet to be put to full use. All that would need is entering your email-adress in the "subscribe to myraworld" field and a click.
Any concerns that this might divert attention away from the LJ-community myraworld is unfounded, it will just be an extra layer of depth and an extra means for communication, in which I don't expect high mmail frequency any time soon (and if there were, there's always the daily-digest option). So if it's just the mailinglist you want to join (and I invite you to join either the group or the list), do this down here: