There is no way to add Things to a Group. There is no way to link an Item to the Online status of a Thing.
There is a way to trigger a Rule based on the Online/Offline status of an Item. https://www.openhab.org/docs/configuration/rules-dsl.html#thing-based-triggers
Thing <thingUID> changed to OFFLINE
It is not possible to get the Thing linked to an Item as far as I know, unless you directly query the REST API from your Rule.
But Design Patterns: Generic Is Alive or even just https://www.openhab.org/addons/bindings/expire1/ might be sufficient.