If you're making plans with friends, would you tell them if other people were coming too?
Or, if someone asked, "Fancy going for a drink on Friday?", would you assume it was just the two of you/your families, or would you expect there to be a group?
Personally, if I'm organising something, I'd always mention if it's a group thing. I'd say something like, "Just seeing if a few people fancy drinks on Friday," or words to that effect. But it doesn't seem unusual to turn up to drinks, dinner, or a kids' playdate and discover there are several people there, sometimes people I've never met. It's made me wonder whether I'm the odd one out for giving people a heads-up. If I'd made plans with someone and then later extended the invite to another I'd go back and let the original invitee know there would be others there.
Knowing others will be there wouldn't stop me going, but I'm quite anxious meeting new people so a heads up gives me a chance to prepare myself mentally, think of a few conversation starters, and maybe make a bit more effort with my appearance, purely for my own confidence.
I've started actively asking whether anyone else will be there, and I'm amazed by how often the answer is yes, even though the organiser hadn't planned to mention it.
So, is the onus on the invitee to ask if others are invited, or should the organiser be giving people a heads-up?
And, if you're someone who doesn't tell guests they're not the only one/s invited, how come?