OP - please see BlackBerry Messenger - problem adding contacts
There are currently some random "issues" with PIN-PIN and therefore BBM. Several people still trying to find fixes.
As for BES, IT Policy needs to be:
Device-Only Items > Allow Peer-to-Peer Messages > True or Default
Security Policy Group > Disable Peer-to-Peer normal send > False or Default
BlackBerry Messenger Policy Group > Disable BlackBerry Messenger > False or Default
If I think of any more, will update post.