There are two type of Direct Messages you can send to Jam members. You can Direct Message a single user or you can Direct Message an entire group of users or all of your followers.
Direct Message a Single User (@user)
Sending a Direct Message with @user, e.g. @gerrard, will send your message to the member's @user feed (Home: @user) and will generate an email notification to that user if their Direct Messages email preference is enabled. (See Email Preferences).
In the Status Update Publisher, type an @ and begin typing the name of the person you want to reach. A drop down list containing all possible users based on the letters you type will appear and you can select the desired name.
Direct Message to an Entire Group or All your Followers (@@notify)
Sending a Direct Message using the special keyword @@notify will send your message to everyone in a Group (if you are posting to a Group wall or on a feed item or object within a Group) or to all of your followers (if you are posting to your Profile or on an object associated with your Profile). The feed item will appear in the @user feed (Home: @user) of each member messaged and will generate an email notification to the users if their "Direct Message to All Followers (using @@notify tag)" email preference is enabled. (See Email Preferences).
KBA , LOD-SF-PLT , Foundational Capabilities & Tools , How To