How do I add a person in the chatroom to my friends list?

Tap the profile picture of the person to open the mini-profile and select Add Friend. 

OR - In the chatroom, tap the triple-lined menu on the top-right corner and you will see the participants in your chat. 

A question mark will appear on the profile picture of any participant who is not on your friends list. Tap the silhouette button to add the user to your friends list.