The Jewish Grace After Meals (aka "benchin" from the Yiddish) is a series of four blessings, followed by a number of pithy requests, thanks, and other extraneous material. It's a few pages long and takes a few minutes to recite.
Every Jewish household has a drawer full of benchers (small pamphlets with the benchin in them), since that's the one keepsake you have to take home from every Jewish celebration. If you threw a celebration of your own, you probably have a few dozen left over that you've been trying to push on friends and relatives who couldn't come, or weren't invited to, the celebration.
I have a vast collection of benchers from the weddings of once deliriously happy couples who have subsequently divorced. It's my acrimony matrimony collection. I'm thinking of sending it out on tour.
Ashkenazi Jews have a universal melody for benchin that kind of fits most of the paragraphs, but leaves a few that you have to intone in a sing-song manner. A great swath of the Sephardi world has picked up the melody (why, I don't know, since it's pretty awful), but some still use their own special melodies.
Despite the near-universal melody, benchin is not always sung. Sometimes the entire thing is just said (mumbled) under one's breath. If multiple people ate together, they might mumble the whole thing, or they might sing the first paragraph and then mumble the whole thing, or they might sing the entire thing aloud.
There is no standard as to which one is going to happen when you begin to bench. You might start to say it under your breath, only to hear others start singing, at which point you curse under your breath and wait for them to catch up to you so that you can join them. Or you may start singing, only to receive glares from the assembled teenagers who wanted to mumble the benchin quickly so that they can run off to join their friends for the usual game of "I don't know, what do you want to do?"
If less than three are assembled, the mumbled version is most common. If three or more are assembled, a call and response to begin benchin is required. Right after the call and response is finished is where the fun begins.
One person starts to mumble. Another starts to sing strongly, only to fade after a few seconds, looking around to see if someone is joining in. Is this a singing or non-singing household? Another who started mumbling stops short and begins half-heartedly singing along with the second guy. The two of them carry the first paragraph's melody to the end and then quickly look down and mumble the rest. Sometimes, if enough people start the singing or lustily join it, they may carry on singing until the end.
At other houses, what will happen may be understood. My father always sings the entire thing aloud on shabbat and holidays, for instance.
Until last Friday night, I never heard a guest ask what the custom is in the house, before the benchin started. My friend Avraham actually asked our mutual host how much of the benchin they sing before we started. I was flabbergasted. Asking? How weird? Don't you know you're supposed to awkwardly mumble, half-sing, and look around you furtively to see what everyone else is doing? Newbies!
He'll learn soon enough.