Otherwise known as wolfberries, Goji berries are the fruit of the Lycium barbarum and Lycium Chinese and are native to southeastern Europe and Asia.
The berry usually measures between 1-2 cm deep and contains anywhere between 10-60 tiny yellow seeds. Goji berries usually ripen between July to October in the northern hemisphere.
They are usually dried and then cooked before consumption, but are also eaten raw. They are renown for their healthy benefits for humans.

So if they are good for us, can guinea pigs eat Goji berries, and if they can, how much can they eat?
Lets check out the nutritional data and see if we can find out a bit more about them.
Total Fat 1.6g 2%
Saturated Fat 0.0g 0%
Cholesterol 0mg 0%
Sodium 0mg 0%
Carbohydrates 24.0g 8%
Dietary Fiber 3.0g 12%
Sugars 21.0g
Protein 1.0g
Vitamin A 170%
Vitamin C 20%
Calcium 8%
Iron 12%

Read more: Calories in EatRaw – Tibetan Goji Berries, Dried | Nutrition and Health Facts http://caloriecount.about.com/calories-eatraw-tibetan-goji-berries-i119927#ixzz2s0rXuDY1
As you can see goji berries contain quite a bit of calcium and sugar and a hint of fat.
However, they do contain a very nice amount of vitamin c and a huge amount of vitamin a which is great for guinea pigs.
This means that guinea pigs can eat goji berries but they can only eat them raw, and once or twice a week at the most. They are a nice fruit for guinea pigs to eat but because of their sugar and calcium content, shouldn’t be fed frequently to guinea pigs
They can’t be eaten dried which is the state that a lot of people eat them.