Click into the agenda item you would like to attend and click on 'schedule' to add this to your schedule. This way you can personalise your own agenda and receive notifications for your upcoming sessions.
Your personalised agenda will sit within the My Schedule tab. Alternatively, click the profile menu (top right) and select My Schedule.
Click the attendees tab, search for the attendee (either by name, job title or company), select their profile and click chat in the bottom left corner. If the chat button is not available, they have opted out of chat messaging.