Kirchhoff's Current Law at the ground node?
You can use KCL at the ground node.

First, because in many circuits, the "ground" node is not actually connected to earth ground or to anything external circuit at all. It's just another circuit node that we use as reference for measuring potential at other nodes in the circuit.

Second, even when the circuit is externally grounded, there is the cut-set form of KCL. A cut set is any set of branches of a circuit that, if they were cut, would split the circuit into two disconnected circuits (but if any one of them were not cut while the others were, the circuit would remain connected). The cut-set form of KCL is: 


  KCL (cut-set law) For all lumped circuits, for all time t, the algebraic sum of the currents associated with any cut set is equal to zero.


(source) 

What that means is, if you consider all the places where your circuit connects to earth ground, the net current through them is zero. So therefore KCL can be applied to all the places where other elements of your circuit connect to the local ground node.