According to this, the K520 supports up to 16 concurrent users while sporting 2x GK104 based GPUs of power. When fully utilized I figured each user won't be getting much more power than a single midrange GPU, and I can't imagine why Amazon would choose to not keep them fully utilized either. If you have any sources otherwise, I'd love to see them as well.
Regarding the CPU, when creating an instance you can see this: "G2 instances are backed by 1 x NVDIA GRID GPU (Kepler GK104) and 8 x hardware hyperthreads form an Intel Xeon E5-2670". According to [Intel's product page](http://ark.intel.com/products/64595), the processor only has 16 hyperthreads, so 2 users per CPU. My reasoning may be wrong though, I'm not a virtualization expert at all.
I had always assumed that their utilization rates would be much higher than 2 users per physical CPU, even for higher end instances like this one.
The Nvidia GRID GPU almost definitely is shared as well, since that's the entire purpose of the GRID SKU:
http://www.nvidia.ca/object/grid-technology.html
According to this, the K520 supports up to 16 concurrent users while sporting 2x GK104 based GPUs of power. When fully utilized I figured each user won't be getting much more power than a single midrange GPU, and I can't imagine why Amazon would choose to not keep them fully utilized either. If you have any sources otherwise, I'd love to see them as well.