You can invite students to join your Blackboard Collaborate session by providing them with a guest link. The link for a session you have created can be found and distributed via the following steps:
Navigate to the Blackboard Collaborate Ultra course tool (it can be accessed from the Other Bb Tools menu item)
Create a new session by clicking on Create Session
Click on the ellipsis button (circle with three dots in it)
Click on Copy guest link
The link has now been copied to your clipboard. Paste this link in an announcement, an email, or a course content area you have set up to inform students about the Collaborate session.