As of 4.0, users can now tag members of a space in chat messages using mentions. Ping a user by typing the '@' symbol, or selecting the '@' symbol to the left of the chat field. This will bring up a list of all users who can be pinged in the space. You can navigate through the listed users via the ↑ Up Arrow or ↓ Down Arrow on your keyboard or scrolling through the pop-up box. Alternatively you can narrow your listed users by typing in letters that correspond to the username you wish to ping. For example typing '@Joh' would narrow your selection down to anyone with '@Joh' in their username such as '@John'. You can then select the username or hit the enter key to add the mention to your message.
If you need to cancel the mention, you can hit the 'Esc' key.
Sending a message with a mention will cause the mentioned users system to generate a system notification alerting them that they have been pinged.