As far as I know there is no notification operation in the REST API: https://api2.clarizen.com/V2.0/services
However it would be very easy to build a workflow rule for this. Basically I would create a toggle custom field in a relevant entity (i.e. in a group containing the users to be notified) or at the organization level and a text area custom field for the notification message. Through the REST API I would update the toggle to true and set the message and then had a workflow rule that triggers when the toggle is changed and set to true. In the workflow rule I would execute a notification step iterating through the list of users to be notified with "Run on" (they have to be related to something pre-configured: members of a group, resources of a task, team members of a case...). The workflow rule also has to take care of setting the flag back to false and clear the notification message.