You can run a simple mailing list, with a small and fixed list of recipients, just using the alias file for a domain. This is simply done by having a local part in your domain expanding to multiple recipients.
For example, you could edit the file /etc/mail/mydomain.com, and add the following lines:
list: user1@domain1.com,
user2@domain2.com,
user3@domain3.com
This will mean that any mail to list@mydomain.com will go to user1@domain1.com, user2@domain2.com, and user3@domain3.com
If you want to have more than a handful of recipients, or if you want people to be able to add and remove themselves without your intervention, then you'll need proper mailing list software, so read on.
First you need to pick a name for your list. Unfortunately, although your mailing list will be associated with a particular domain, then name still has to be unique across all mailing lists hosted on the server. Look at http://bear.popebanana.net/mailman/admin for a list of extant lists, and pick a name that's unused.
Then you should type "new-mailing-list" at a bear ssh prompt, and give it the name you chose earlier, and your email address (or, if you want someone else to be the list manager instead of you, then their email address), and a list manager's password.
It will output a few lines of stuff, which you should cut and paste into the aliases file for your domain. These will be used to tell the system to send any mail to the list address into the mailing list program.
However, you need to associate your mailing list with your domain. Go back to http://bear.popebanana.net/mailman/admin and select your list which should now have appeared here. After entering your list manager's password, you will get access to several pages of settings about your mailing list. At the bottom of the first page are two options you need to change.
Firstly, "Host name this list prefers" has to match the name of the aliases file to which you added that stuff earlier, something like "mydomain.com".
Secondly, "Base URL for Mailman web interface" should be the URL used for your mailing list interface; it doesn't _have_ to be in the same domain as the mailing list itself, but almost always it will be. Unless you know what you're doing, set it to "http://www.mydomain.com/mailman".
Note that you're not automatically subscribed to your own lists, so you probably want to subscribe yourself next.
By going to "http://www.mydomain.com/mailman/listinfo", your users can get a list of all the mailing lists in your domain. Clicking on a list name will go to a page with information about the list.
To subscribe they should follow the simple instructions on that page. This will send them a mail which they need to reply to to confirm their subscription (the list manager can remove the need for confirmation, but there are good reasons for keeping it on). Once they have replied, they will be subscribed.
They can do everything else they need to do - unsubscribe, or change their subscription options - by logging in using their email address and password.
Go back to the the admin URL mentioned earlier to change the settings for your list or perform other administrative tasks on it.
Your mailing list is automatically archived. By default the archives are public, though you can chose to make them private or not to have any archives at all using the admin interface. To access public archives of your list, people should go to "http://www.mydomain.com/mailman/archive/mylist/" where mylist is the name of your mailing list.
Unfortunately, you can't delete a mailing list yourself. I can't be bothered to put in the checking required to ensure you don't delete someone else's mailing lists, so since it isn't something you need to do very often, you'll have to mail me and get me to do it for you. Sorry.