Hi Joe,
You need to write a Java Policy of alfresco. Your policy implementation class will need to implement <java>NodeServicePolicies.OnCreateNodePolicy</java> interface and write its <java>onCreateNode()</java>. Now whenever a new node is created, this <java>onCreateNode()</java> will be called. So in this method you need to
1) Get site name in which document has been uploaded
2) Get all members of the site and make a list of members excluding current user/uploader/author
3) Once you have members list, get their registered email id(Get PersonNode from the username and from PersonNode get their email id)
4) you can use Java Mail API to send mail to all of them.
In Policy class of the Alfresco, you also can specify if you want to send the mail when some special type(custom type) of document or document with special aspect is uploaded. You will need to define this in Policy class's <java>init()</java>.