Dip the rim of a rock glass into lime juice and then into margarita salt. Fill with ice, being careful not to knock off the salt. Set aside.
Add cranberry juice, tequila, Grand Marnier, and lime juice to a cocktail shaker filled with ice. Shake for 20 seconds. Add agave syrup as needed and shake again until desired sweetness is reached. Pour into prepared glass.
Garnish with a lime slice and fresh cranberries, if desired. Serve immediately.
I don't always use agave syrup to this cocktail. It depends on how sweet the cranberry juice is. You can use sweet and sour mix in place of lime juice if you want your margarita less tart. The lime and cranberry are great together though!For a large batch: In a 2.5-quart pitcher combine 2.5 cups cranberry juice, 2.5 cups tequila, 1.25 cups Grand Marinier, and 2.5 cups lime juice. Stir and sweeten with agave syrup as needed. Add lime slices and fresh cranberries to the pitcher, if desired. Makes 10 margaritas.