Any one can join, though if you post off topic you may be scolded by your peers. Commands might include requests to subscribe to the list, to unsubscribe, etc. Majordomo is the list software used on the University of Oregon campus for local mailing lists such as class lists. However, if you decide to subscribe to a mailing list managed at some other site you may find yourself interacting with some other flavor of mailing list software, with slightly different commands and features. Here is a page listing Commands for most of the major types of lists.
Most mailing lists allow the members of the list to post messages to the list.
You should use the subject line. Remember that your message is going to all members of the group, so be brief and to the point. Remember if you are replying to a message from the group to be very careful to make the correct choices about replying to all recepients or to an individual. Many people have sent messages to a a large group that were meant for only one person. The results of this can be very embarrassing or serious.