Here's a few sources for recipes that are similar to what you get at Sweetwaters when you order a Ginger-Lemon Tea (one of my favorites).
From Coppertop Kitchen (a blog by someone who was a Sweetwaters barista): Ginger Lemon Tea
2 cups fresh lemon juice
1 3- by 1-inch piece of fresh ginger, peeled and sliced thinly
10 cups water
1 cup honey
Place lemon juice, ginger, and water in a large saucepan over medium heat, and bring to a boil. Boil for a couple of minutes, cover and remove from heat, and steep at least 30 minutes.
From Food Network, Lemon Ginger Tea
1 quart water
Juice of 1 lemon
1/4 cup freshly grated ginger
1/4 cup honey
Thin slices of lemon and lime for garnish
In a saucepan bring water to a boil. Add lemon juice, the squeezed lemon, and ginger. Let steep about 20 minutes. Stir in honey. Line a strainer with a thin wet cloth and strain tea into a pitcher.
From The Ginger People's newsletter, writing about Sweetwaters:
They initially juiced their own ginger for the Ginger Lemon Tea but its increasing popularity eventually caused them to look for a commercially available ginger ingredient. After considering several options, they decided on The Ginger People's ginger juice because of its high quality and intensely true ginger flavor. Their secret recipe, which now includes our ginger juice, is served either hot or cold, and is one of their top selling drinks.
So there you go, the secret ingredient – you'll have to buy it in industrial quantities, or juice your own.