Is it possible add a group to selection (not selection to a group) in the in 3d viewport? Is it possible to add a object group to selection (not selection to a object group) in the 3d viewport? I know it can be done in the outliner, but I would rather some sort of shift click function in the viewport similar to how add objects to selection is done in the 3d viewport. Thanks.
Would it be Shift+G > Select Grouped > Group > Group Name?

After executing the operator, on the bottom of the tools panel or pressing F6 you can open the operator properties and tick the Extend option to extend current selection