My job share partner was telling me about her mother-in-laws guinea pigs recently. They arrived in the garden from a house further up the street and made a home under her shed. They were never claimed by the neighbour so they stayed living under the shed for years - all year around. The MIL would put out bits of blanket for them in the coldest weather which they dragged under the shed and the guineas started coming to the back door and wheeking for food. They were given odd bits of veg and slices of bread which they took back to their home but apart from that, grass and other plants in the garden were their main diet (and as this is the UK, I'm sure much of that grass would have been wet!).
I'm not suggesting this is a good idea obviously, but that's an example of 2 guineas living properly outdoors with minimal support from humans for a number of years apparently quite happily!