Once you are subscribed to a group, to send messages to the group, just address them to groupname@googlegroups.com.

Keep in mind that some of the lists(like the stmhsa-general list) are moderated, so your post will not be delivered until a list administrator approves it.

Also, you can view all posts to a list by logging in to Google Groups and clicking on the group name under the “My Groups” drop down menu.