If you want citrus/grapefruit up the wazoo, use LOTS of hops between 15 and 5 minutes and dryhop like a mother.
Use Amarillo/Centennial/Cascade/Simcoe/Columbus/Zeus/Tomahawk All of these will give a lot of citrus, Amarillo/Simcoe being a classic combination.
The key is using lots of hops to maximize the flavor and aroma.
But if you're keen on putting real grapefruit in, don't put juice or even the fruit itself. Just zest some washed (to get rid of the wax..or organic instead) with a Microplane and throw it in a flame out, or in a french press, or in the secondary. Fruit and juice will be too bitter, not flavorful. The oils in the zest is what you want.