The easiest way would probably be to give them a mailto URL pointing
to the listname-subscribe-request address, ie,

  You can subscribe to the MYLIST-L list by clicking
  <a href="mailto:[log in to unmask]">
  here.</a> (If your mail program does not support clickable address
  links, simply send a blank mail message to mylist-l-subscribe-request@
  myserver.example.com to obtain a subscription.)

This requires 1.8d.

Nathan