Where should I add ground pour?
Your board is extremely compact and since most of your components, if not all, are on the top layer, putting another pour on the top layer may do more harm than good.

As Ignacio noted, your crystal positioning is a little concerning.  Give this document from Atmel a good read.  It explains how to position a crystal on a PCB.

With that said, it looks like you don't have many traces that would block out the possibility of copper pours on the bottom layer.  Now you have quite a few traces running in parallel for a bit with each other, if those traces are being switched quite rapidly, you may see some crosstalk between them.  Putting a ground pour on the bottom layer would mitigate this.  Be sure to mind the crystal and don't put the ground plane underneath it.

One thing, to make the bottom layer ground plane more whole, keep the trace connecting to the third drill from the right on the top row on the top layer.  There really isn't any reason to move it to the bottom layer as there is clearly enough space to keep it on top.  Putting it through a via will just induce unnecessary noise.