To subscribe to a list, send a mail to the list server email address, with the subject as subscribe <listname>, or send a mail to <listname>-request@<listdomain> with the subject as subscribe.
For example, to subscribe to a list named engarde-users, which has the domain guardiandigital.com, send an email to the list server email address with the following line in the subject, or in the body if subject is empty:
subscribe engarde-usersOr, send a mail to engarde-users-request@guardiandigital.com, with the subject as subscribe.