MCP41100 Power supply and grounding ? Analog ? Digital?
The MCP41100 (along with probably every other digipot I've seen) doesn't need nor use an analogue ground. It is a digitally controlled potentiometer and the three connections to the potentiometer are floating and can be connected to your target analogue circuit however you need them to be (within reason).

Here are a couple of examples from the data sheet - these examples show how the device is wired to test for frequency response and capacitance but, the diagrams serve the purpose of showing the 3 pot connections are not needed to be connected to ground (unless you want them to be): -



Note also that this particular device should not have voltages applied to the pot connections that are outside the absolute maximum ratings for the device (-0.6V to Vdd+1V). If in doubt, read the data sheet.