Thanks for the help! Here's some of the options I'm exploring:
The Seasons expansion pack enables the grand meal and singing holiday songs. And holidays can have a singing tradition, but I'm not sure I want this tied to a holiday.
I'm also looking at the Holiday Celebration pack. I don't see anything about singing on the Fandom page, but I've got the pack. I haven't done much with the pack yet, but I can experiment with the stuff it adds. I'd like to have group singing without being tied to a particular holiday object though.
In Adventure Awaits or Outdoor Retreat, it seems that group singing is tied to the campfire, rather than something that can be done anywhere, as long as someone can play the guitar.
I don't see anything about singing in the Fandom page for Get Together, but it does add social clubs, which could definitely help.
City Living adds the Singing skill, but that seems performance oriented, rather than something that could be used to lead others in group singing.