Already answered in microsoft.public.access.forms where you asked 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, I believe that you can
type the names of the various groups into the Newsgroup box. Separate each
newsgroup name with a semicolon.

Note that it's generally consider to be A Bad Thing to cross-post to more
than about 2 or 3 newsgroups. (In fact, at
http://www.microsoft.com/presspass/features/2001/Mar01/Mar27pmvp.asp
Microsoft suggests that "One group will suffice")