By default, team members have viewer (read-only) rights to any session under the Team’s workspace. If you’d like one of your team members (or anyone else) to collaborate with you on a session, you’ll need to invite them as a collaborator and assign them editor rights.

You can do this if you open any session plan and click on the 'Share' button at the top of the Session Planner:

An activity indicator signals if your collaborator is online and viewing the session. If this is the case, then the thumbnail image of the user will be marked with a green edge.