There are three areas from which you can send messages: Your notifications inbox, from events, and from credentials.
You can send general messages to any number of users right from your inbox
For more information on how to send messages from your inbox, click here.
For information on how to view your sent messages from your notification inbox. click here.
You can also send messages directly from the Manage Events module. From here, you can send messages to all users enrolled in an event, just to those that attended, or just to those that were absent.
For information on how to send messages for an upcoming event, click here.
For information on how to send messages for a past event, click here.
For help with sending a message from an event with no enrolled users, click here.
You also have the ability to send messages from a credential. You can send them to individual users within a credential, or all users in a credential.
To do so, click the users gear from within Manage Credentials. From there, click the "Message Users" option in the "Actions" dropdown to send a message to multiple users, or click the mail icon next to the user you want to message to send an individual message.
For more information on how to send messages to users in a credential, click here.