You can cross-post your message to more than one newsgroup if your message is relevant to more than one group. Cross-posting is easy to do, and it is similar to emailing to more than one person. Simply put a comma between each newsgroup name you wish to post to in the address field.
However, remember that excessive cross-posting is considered bad form without a good reason, because it multiplies the traffic in Usenet several times without adding any new content. In the extreme case, if everybody cross-posted every message to every group, then every group would look exactly the same.
You should cross-post only when it is really needed, and usually not to more than three groups. All cross-posted groups should be closely related in subject. It is considered bad netiquette to cross-post a message to totally unrelated groups, and could cause you to receive a large number of complaints by email.
Because cross-posting of commercial advertisements has become a problem, some news servers will ignore any message which is cross-posted to too many groups. Depending on the server configuration, somewhere between five and fifteen is the usual limit. When you cross-post an article to more than one group, you may want to specify that all responses should be directed to a single group.