Subscription is very straightforward and of course it's free.
Go to the P2P home page and click on the subject area you are interested in. On the next page you will see a list of the mailing lists under that topic heading. Click on the list you wish to join. You will see a list of recent messages posted to that list. You will then be required to supply your email, a name by which you will be identified on the list, a password, and finally several options including whether to receive the list messages as they are posted or as a daily digest.