The previous version of the Collaborate interface on Blackboard provided an "Allow Guests" link next on the button for both the course and instructor rooms.
In the current version, you can allow non-Blackboard users to attend by selecting to "Allow Guests" in the "Room Attributes" section, when creating or setting up the room.
The new interface to the Collaborate system on Balckboard has also added a "Learn More" link to find out how to upload Power Point files into Collaborate.