Irene thickened up the pie with tapioca starch (though any kind of starch can be used), because let's face it, grapes are full of liquid and the pie fillings comes out super runny.
Depending on how jammy you like your pie (or runny), you can use flour, tapioca starch, or tapioca pearls. Personally, I like my filling on the firmer side; once a pie starts falling apart, I feel like I'm losing the ratio of fruit to crust, and then I'm thinking about how I'd rather just have a crumble or crisp and some buttery brown sugar tidbits.
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