On the Device Manager page, open a specific device to edit (a device action button > Edit). Then, on the device settings page, go to the Alerts tab. In the Send Alerts to Group(s) section, click the Add Group button and select the check boxes related to the necessary group.
You also can configure the Escalation option allows you to start alerting if downtime remains for longer then a specific period of time.
See also: Configuring alerts settings