Mix Together: Combine the cranberry juice cocktail and triple sec in a large pitcher. You can scale up the quantities based on the number of guests.
Refrigerate: Store this mixture in the refrigerator until you're ready to serve. This can be done several hours ahead of time, or even the night before.
Chill the Sparkling Wine: Make sure your unopened bubbly is thoroughly chilled.
Combine: Right before serving, gently stir in the chilled sparkling wine into the pitcher with the juice mixture. Doing this too early can cause the wine to lose its effervescence.
Tips for a mimosa bar:
You can also make a "mimosa bar" by leaving the cranberry juice cocktail and triple sec mixture in the pitcher and keeping the chilled bubbly ready next to it.
Guests can pour from the pitcher and then top with the bubbly. Although, some people can be a little heavy handed with the bubbly, so I'd have an extra chilled bottle or two in the fridge just in case.