(Barbershop singing isn't just harmony, but the use of four different voices singing harmony. In other words, not four tenors harmonizing, but an alto, tenor, baritone and bass singing harmony.)
This is my own favorite group, the Ambassadors of Harmony, out of St. Charles, Missouri, near St. Louis, lead by a fellow named Dr. Jim Henry. They broke through in 2009, ending a very long streak by another chrous called Masters of Harmony, which had won something like 10 years in a row. (Not exactly that, but close). They've now won Best Chorus group at the International Barbershop competition -- where Mark's video above took place -- four times.
One of things I love about them is not only their wonderful, textured singing and clever arrangements, but their showmanship. This is that 2009 performance that put them on the map, not only winning but apparently with the highest score ever recorded up to that time, with a song you'll easily recognize. (It's from a show known for its barbershop singing, though oddly this is not one of them!) And from the thunderous reaction of the audience at the end, you'll hear why they won. The performance is great fun from the beginning, but if you're not a fan of barbershop stick with it because things kick up a notch later on.
I've posted this before, but years ago, and it bears repeating.