I have my guinea pigs inside, they are very very prone to respiratory problems if they are kept outside, also, if they are kept outside they are not to be brought inside to have a cuddle etc as it can make them ill.
If you do keep them outside they need to be kept in an elevated cage with a covering to protect them from wind etc.
In the summer, you need to make sure that they are kept in the shade on hot days as they are susceptible to heat stroke, don't put any of these added vitamin c drops to the water as it can make the water taste funny and they won't drink.
Don't give them normal rabbit food as it can make them is, they are best having guinea deluxe, its the same price and you can get it from vets. As long as they also have a constant supply of fresh water and hay (alfalfa hay is one of the best up until they are 6 months old, then give it as an occasional treat).
They love fresh fruit and veggies too, don't give them potatoes though as there is too much starch.
Every day my 3 get
Cucumber (with skin off as its too waxy)
A slice of orange
A baby carrot
Cherry Tomato
Cauliflower
Brocolli
Any kind of melon
Lettuce (any but romaine and iceberg)
Dandelion leaves and grass
As you can tell, I love guinea pigs and know quite alot about them , if you need any more help, feel free to ask