Already answered in another newsgroup to which you posted the same question.

If you feel you need to post to more than one group (HINT: it's seldom
necessary), please have the courtesy to cross-post (send the one message to
all groups at once), rather than multi-post (send individual messages to
each group). In this way, all responses to your post will be available
together, regardless of what group the responder was in, and the rest of us
won't have to read your post multiple times. (It also uses fewer server
resources)

If you're using Microsoft's web interface to post, you should see an
"Advanced Options" link at the bottom of the page. You can type the names of
the various groups into the Newsgroup box, separating each newsgroup name
with a semicolon.