Cut a thin slice from each end of each grapefruit. Using a very sharp utility knife or a serrated knife and cutting from the top of the fruit down, cut off the grapefruit peel and the white membrane. (Or, cut around the fruit in a spiral.) Working over a bowl to catch the juices, cut between one fruit section and the membrane. Cut to the center of the fruit. Turn the knife and slide it up the other side of the section next to the membrane; repeat to make sections. Remove any seeds from the fruit sections. Set grapefruit sections aside (should have about 3 cups).
Add enough water to the reserved grapefruit juice to measure 1 cup. In a medium saucepan combine the 1 cup grapefruit juice, the sugar, and marmalade. Heat to boiling, stirring to dissolve sugar. Add cranberries; cook and stir for 5 to 8 minutes or until skins pop. Remove from heat; cool. Stir grapefruit into cranberry mixture. Cover and refrigerate until thoroughly chilled.
Just before serving, stir sliced bananas into chilled grapefruit mixture. Makes 10 side-dish servings.