There is the TAB menu that says who is in the map you’re currently in, but there isn’t a message that tells when somebody enters or leaves the map, also there isn’t a way to distinguish if a friend of yours is just in the website or playing inside a map, and you could also add shortcut to the map your friend is currently playing.
This feature was already in-game, but for some reason Ink decided to take it out for a moment, and i guess it will come back during another version of the game.
Probably the shortcut for entering the same map your friend is playing also can be planned to appear during a future website version too.