It worked for bananas, why not make a grapefruit wine?
1. Cut grapefruits into slices leaving peel on
2. Boil and add a shit-ton of sugar
3. Put contents in a fermenter and add some wine yeast
4. Leave it sit for a year because the internet said so ok?
5. Bottle
6. Blow minds at swap
Ha! That banana wine was surprisingly popular.
Laxation - the other option is to juice a heap of grapefruit, freeze the juice in a sanitised container (freezing should kill most bugs) and then add the defrosted juice to a partially fermented beer. The other option is taking some zest, steep it in vodka for a week or so and add both to finished fermenting beer and leave for a few days before bottling.
I'm planning on grapefruit zesting a small portion of sour when I get round to bottling one of my batches currently on the go.