You need BES. Nothing else will do it. No hack of any kind will allow it. BES or nothing. BES Express is free for the first user, $109 each additional user, and if you are lucky, and buying 5 blackberries soon, you can get 5 users for free.
IF your buying 20 blackberries, get 20 for free (if you buy all at once)
check with your carrier to see pricing and offers available for BES.
And i'll repeat -- NO bes? NO WIRELESS CALENDAR! PERIOD! =)