To follow someone and be notified when they post, click on the icon at the top-right of their post in the activity feed that looks like a person with a plus sign. The icon will change to two people to indicate that you are now following the person.
You can also click on the 'Attendees' menu section of the app. Search for attendees that you want to follow by scrolling through the attendee list or by searching their names using the magnifying glass icon on the top part of the screen. Click on the icon with the plus sign to follow the person.