I’ve led many a singalong. Some better, some worse. The good ones typically have a couple of things in common:

1. Multiple guitars.
2. 3+ really enthusiastic singers.
3. Enough participants. 5+ necessary, 10+ starting to get ideal, 100+ is unnecessary but magical.
4. 50%+ of the group know all words in the choruses of 50%+ of songs played.
5. Songs that few people know are quit very quickly.
6. Participants are made to feel included (ideally central, but at least in-on-the-fun).

On the other hand, there are many ways the singalong might fall apart:

1. Any one of the above conditions being violated.
2. The presence of try-hard pianists who have learned popular songs or can play melodies by ear. Instrumentalists need to follow the participants (who don’t always have perfect sense of tempo etc.), not the other way around.
3. Awkward group of instruments.
   1. Adding drums or a keyboard to a single guitar leads to synchronization issues/participant confusion.
   2. But in a big enough group (5+ instrumentalists), you more or less can’t go wrong by adding any marginal instrument.
   3. Including more instruments that don’t add bass-lines – ukulele, violin, I’m guessing saxophone – is basically always good, even with insufficient guitars.

Some factors that are perhaps surprisingly unimportant:

1. Instrumentalist skill.
2. Participant skill.
   1. With one important exception: incongruous mashups, where it’s crucial that at least 2 singers can confidently sing contemporaneously/on top of one another. This arrangement can work beautifully, but totally totally flops if you don’t have confident singers on each song – by default, participants assume that the group will only sing songs sequentially.
3. Mechanism for choosing songs.
   1. I prefer either launching into songs and seeing what sticks, or democracy/having participants vote on a running list. But not a big deal.