Rigid body settings to a group as a whole I would like to apply rigid body physics to a grouped set of objects, but would not like that set of objects to be dismantled during the simulation (preserving their relative positions). 

For example, imagine two spheres forming a "water molecule":



And I want to bounce such molecules in my simulation without breaking them apart. How do I go about doing that in Blender?
Join objects together. Select objects you want to join then press Ctrl+J.