I sometimes use sweet potato for beer in a similar way as I do pumpkin. Just clean it up, shove it in the oven until it caramelizes a bit, mash it with a fork or whatever is at hand and add it to the mash - I usually use 1-2 kg (~2-4 lbs) for a 20L (5 gal?) batch. And from then on it's brewing as usual. Gives a nice colour to the beer though I can't say I've noticed any flavour.

Read about being able to mash them on their own, but I've yet to try that, not sure how I would go about it.