To start getting notifications in Chats (and optionally in your email) about absences of a certain person or all members of a certain team:
To get notified about an individual person, follow that person by opening their profile and clicking Follow next to their name. You can also follow a member from his/her team page, by moving your cursor to the member's card and clicking .
To get notified about all members of a specific team, add that team to Favorites by going to the team's page and clicking next to the team name.
You can tune your absence notifications on the Subscriptions page: