In a blender, blend grapes until pureed. Strain skins.
Line up 4 (25 ounce) bottles with flip top lids or other air tight bottles.
Pour ½ cup grape puree into each 25 ounce bottle.
Using a funnel, pour freshly made homemade kombucha into each 25 ounce bottle leaving 1-2" head space.
Let bottles sit out (at 75° - 85°) 3-5 days, making sure to pop the lids each day to let out the pressure of the carbon dioxide. Taste each day to test its flavor and fizziness.
Notes
If you don't want grape puree in your kombucha you can chop it up and smush it into each bottle, then strain after fermenting, if desired.To make it sweeter, add maple syrup or your favorite sweetener.