You have the benefit of writing in your native tongue, many do not. Subtle humor tends to get lost so take steps to make sure that people realize you are trying to be funny. The net has developed a symbol called the smiley face. See Acronyms and smileys for more information about these Emoticons. Pay attention to the reply address.
Discussion groups vary considerably in their configuration.
Using the reply function on your email program can send your reply directly to the person who posted the original message or it can send your response to the entire group. For example, if the original message is asking who will be attending the dinner on Saturday, make very sure that your response goes to the originator of the message and not back out to all of the people on the list. Get to know the culture of the group. Before you post a message, it is a good idea to have an understanding of the way the group behaves. Avoid embarassment by understanding the culture of the list before you begin posting.