When adding a file to the vault you will see two fields for alerting and sharing a file with other users.

Visible To: This displays the file within the user's vault so that they can view it at any time, but does not create a specific alert for them.
Alert Users: This will display the file within the user's vault and also send them an alert, which shows up on their Action Plan & Alerts Screen.
Send Emails?: This will send an email notification to all alerted users.

Selecting either field will display a list of user groups.

ou can select a user group, or if you would like to select a specific user, pressing the next to the user group will display a list of users within that user group. 

You can then select one or more individual users, and repeat this process to select other users or user groups.