Hi Sally, 

Your friend is looking out for you, and they are correct that the safe way to store your grape juice at room temperature would be to process it in a boiling water canner. Yeasts and other microorganisms love to grow in juice (it's so full of sugar!), and the canning process ensures that you've knocked down the bacterial population to prevent spoilage and to kill any pathogens.

To can grape juice, you start with hot juice (either out of our steam juicer or heated on the stove). You can add sugar if you like, or pour hot juice into sterilized jars (pints, quarts, or half-gallons). Because juice processing times are so short, you do need to pre-sterilize your jars by boiling them for 10 minutes. 

Once you have your sterilized jars filled, adjust your two piece lids and process in a boiling water canner. The level of boiling water should be 1-2 inches above the tops of the jar. Process 5 minutes for quart jars or smaller; process 10 minutes for half gallon jars. Full instructions are online here: https://nchfp.uga.edu/how/can/canning-fruits-and-fruit-products/grape-juice/

The method you used to store your juice is an older style of canning that does effectively seal the jars and prevent any yeasts or bacteria from entering the juice after sealing, but it does not fully sterilize the surface of the jars and lids. Any bacteria or yeast that were on the lids before you started could be in the juice and would cause spoilage. This method has not been recommended for many years for this reason.

If you don't want to have to go through the canning process in the future, you can always refrigerate juice without canning, or you can of course freeze it.

Thanks for using Ask Extension!

Jared