You can "mention" someone by typing @ followed by their username, for example @john. Enter @ and start typing the username or a portion of their name or email address; a drop-down list will appear where you can select the desired user.
In the Browser/Desktop interface, a user can be selected from the list by clicking on the username or using the keyboard arrow keys to scroll up and down to the desired entry. Then select the user by pressing the Tab or Enter key.
If the user is not currently a member of that room, they will be invited and automatically joined to the room. (In the Mobile interface, this is the method for inviting users.)
When someone is mentioned in a room, they will get a desktop notification and also a badge count beside the room name if they are not already in the room.
Each of your mentions are also listed in the Mentions list when you click on located on the Right Icon Menu or when you select it from the Actions/Channel Details screen on the Mobility interface.
The behavior of each of the three alerts is the same regardless of whether the other users are using the desktop, web browser, or mobility client interface, with the exception that the mobility interface does not display a red banner.
Similar to "@ mentions" you can use '#' to mention a room, so that users can click the room name to be switched to that room.