On the website and in Live:
https://support.chess.com/customer/en/portal/articles/1444899-how-do-i-send-a-friend-request-
Or you can view their profile and the Add Friend icon is the same there.
If you are on mobile and you want to try to friend request a player you have played before, you can go into the game archive, select the game, tap their picture and in the main menu there (left-most with three lines, choose Add Friend.
On the app you can also go to Friends and type in a Username if you know who you want to add, then click the orange +
Friendss.???