Yes, Guinea pigs can eat seedless grapes. You probably know this, but since it is a fruit, it is high in natural sugar so should be fed only 2-3 times a week. They are perfect for rewards if you are training them tricks!
a link to the pdf of the charts is on the page also for easy print out ability
according to Ly's chart grapes (25g)
may be fed frequently (blue colour coding)
1 to 4 times per week.
Red or Green and Seedless
The charts are found at the forum menu of diet and nutrition in the sticky notes section - you will also find there sample menu's for ease of use.
I recall reading elsewhere on the forum that 2 grapes at a time is your safe limit. Like Gabbels said watch the sugar, especially, I'd think, if you happen to be feeding other sugary feeds at the same times of the grapes.