Quick Select Object In Collection
With this add-on you can select all objects in a collections with just one keystroke in the viewport: Shift-CTRL-C
Of course you can change this shortcut to whatever you want.
1) select an object (or multiple)
2) press Shitf-CTRL-C
3) all objects of these collections will be selected in which this object is contained
Exception: The collections with the name "RigidBodyWorld" will be ignored.