It must be remembered that guinea pigs have a delicate digestive system and any attempts to introduce vegetables should be done so gradually. If you do not know if your piggies have ever had fresh foods, assume they haven't and start right from the beginning.
Start off by choosing ONE vegetable that isn't very watery, so celery leaves are good starter, and totally avoid watery vegetables like cucumber and all fruits for now. Offer a small piece, no more than a few mouthfuls in the morning/evening and if there appears to be no diarrhoea, do so again the next morning/evening, using the same vegetable. Try this for several days and so long as there are no problems you can gradually increase the amount. If any diarrhoea appears, leave for 3 days and start again, if it occurs for the second time leave for another 2-3 days and start from the beginning. If all goes well with no problems for over a week, you can try one new vegetable and so long as their are no problems you can continue to feed both types, still gradually increasing the amounts and stopping for 48-72 hours on the first sign if diarrhoea. Then you can add in different types off the safe food list.