A little (informal) history. In the 1700s, churches in New England were short on money for luxuries like musical instruments -- an organ or even a piano was out of the question for most. Some enterprising composers decided that if they could write easy-to-learn religious music that was richer than just basic melodies, this could solve the church's problem and possibly make some money for the composers. These composers were called "singing masters" and travelled from place to place holding "singing schools" in which they'd teach people how to read music using special systems of notation that they'd developed. If a singing school that lasted several evenings then let them sell a bunch of books to the church, it was a win-win situation. One of the most successful notation systems was the "four shape" system: note heads, instead of being ovals, could be oval, triangle, rectangle, or diamond shaped, and each note-shape had a name: triangle was Fa; oval was Sol, rectangle was La, and Diamond was Mi. Singers learning a tune for the first time would sing the notes using these shape-words, so they might sing "Sol-Sol-La-Sol Faaaaa-Miiiiii" as the starting notes of the tune we know as "Happy Birthday". Then, having learned the tune, they'd sing the actual hymnal words instead.
Why the shapes and the names? Think about "Happy Birthday" -- when someone wants a group of people to sing that song, they hum loudly, or perhaps sing "haaaaaaa" as a way to get everyone starting on the same pitch before moving on to "aaappy Birth-day to yooooooouuu", right? And it doesn't really matter very much what pitch they choose to start on -- a pitch that works well for a class of 10-year-old children might not be ideal for a group of adults and vice versa, but everyone knows, after the first note, what they're trying to sing next, right? The very last note of "Happy Birthday," written in the 4-shape system, is a triangle, or Fa. The actual pitch that a group sings for "Fa" isn't always the same -- think of the adults vs 10-year-olds -- but within a single group, we initially all agree was sound will be "Fa" and then we sing. Musicians who are certain that the second-to-top space in a G-clef line of music represents a frequency of 440 Hz often find this unnerving, but those who have never sung anything fancier than Happy Birthday find the notion easy to grasp.
Back to the singing schools! The shape-note singing tradition spread throughout New England, into New York, Pennsylvania and Ohio, and to the South. By the time it was spreading South, many New England towns had accumulated some wealth and bought a piano or small organ, and the shape-note singing tradition started to fade. But in that South, it took root and continues to this day -- there are churches for which the tunebook we sing from -- The Sacred Harp -- is the hymnal.
What about the "harp"? The sacred "harp" refers to the human voice; there are no (physical) harps involved in the music.
What does this music sound like? The Sacred Harp has almost no dynamic markings (the things that tell you "this part is loud" or "this part is quiet"), and the music is typically sung with full voice. A YouTube search for "Sacred Harp" will give you a feel for what the music is like and how it's sung. Most of the videos you'll see are from "all day singings," which are extended get-togethers where singers gather from a wide area and will sing many songs over the course of 5 or 6 hours, with a break for a noontime potluck meal. "Local singings" are often much smaller, but they give folks practice with the music, and help them develop confidence in leading a tune.
How do the shapes help? To be honest, for many beginning singers, the shapes are frustrating and distracting. But once you learn the names of the shapes, it's nice to learn a new song in stages: the tune (using shape-names) comes first; then you sing it again adding the words. And after a bit of practice, something magical happens: you start to learn a new tune, and you see that the next note is Sol, and you realize that you know what that note has to sound like! Suddenly each shape is helping guide you to sing the right next note. And then you realize that you have a book with more than 500 tunes, any one of which you can sing even if you've never sung it before, and feel confident that you'll be 95% right in what you sing!
Who organizes things? We all do! There's no choir director. At most singings, each person in turn is given a chance to lead a tune that they pick (or to opt not to lead). During a singing, we never sing a particular tune more than once, so if you lead something and it sounds terrible, we all just chalk it up to it being bad luck, and maybe we'll try again next week. No one will stop the singing and say, "Let's go back to the third measure and tenors, be sure to get that E on the third beat correct!"
Can anyone do this? Absolutely! One singer I know was told by his family that he really shouldn't sing because he was no good at it; he's now an accomplished and enthusiastic shape-note singer. When new singers show up at our local singings, we often begin with a short 'singing school' session in which we go over the basics; we then suggest that the new person sit next to someone more experienced and try their best to learn a couple of the tunes we sing, and when they come back the next time, learn a few more, and soon they're on their way.
Why would I do this? Some people use shape-note singing as a form of Christian religious practice. (Other singers are atheists, Jews, Muslims, Buddhists, ...) Some find the full-voice singing cathartic, and welcome a chance to do something primal at the end of a day. Some love the musical aesthetic of the Sacred Harp. Some find the textual content --- there's a lot of death and misery and possible-redemption in the book --- resonates for them.
I'm really shy, and I don't want to look or sound like an idiot when I come to my first singing. Is there a way I can practice a bit first? Yes, absolutely. Sacred Harp Bremen's website (in both English and German) has a ton of information, and you can listen to a particular 'part' for a tune (most tunes are in 4-part harmony) as a way to learn it. But a much easier way to learn is to come to a singing and join with others; it's how the music has been passed on for more than two centuries, and it works really well. And no one will ever object if you sing a wrong note. They probably won't even hear it! Come to one of our weekly singings and give it a try!