I would like to know how can I add a sensor (a WMI Process sensor, for example) for all devices that are already discovered within a group?


The fastest method would be to clone the sensor in case:

Cloning is available for groups, devices, and sensors. Unlike the results when using the Create Device Template option, a cloned device will contain all objects of the original device, regardless of whether they bring about working sensors or not (which often depends on the settings of the cloned device).

The other option is to use a device template and auto-discovery to update existing devices with new sensors. You can use Multi-edit to perform a template-based auto-discovery on multiple devices at once.


Disclaimer:
The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.