When adding a meeting you see two fields for alerting and sharing the meeting with other users.

Visible To: This displays the meeting within the child's chronology so that the staff member tagged can view it at any time, but does not create a specific alert for them.
Alert Users: This displays the file within the chronology and also send the user an alert, which is displayed on their Action Plan & Alerts Screen.
Send Emails: This sends an email notification to all alerted users.

Selecting either field displays a list of user groups.

You can select a user group, or if you would like to select a specific user, pressing the + next to the user group displays 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.