I have two guinea pigs. They can get a little stinky if the bedding is wet against them for too long. Making sure their cage is big enough and has good absorbent bedding can really help with the stink factor. If they just absolutely reek, then you should have them checked for a UTI. We use a pet shampoo called EarthBath. It is super mild with not fragrance. I sometimes grab it and use it myself because my skin is sensitive. To bathe a guinea pig just get a little plastic tub and put the piggie in there with a small amount of warm water. I wouldn't do it in the sink because they will shake the water off, and you might not want it all over your mirror or toothbrushes.
