I would say that focal length is the most important thing to look at. Somewhere around 35mm-equivalent (so, 23mm or so on APS-C). That's the field of view of most phone cameras these days — because this kind of situation is definitely one they need to cover. Similarly, kit lenses tend to be zooms covering this range, for the same basic reason (although see below for thoughts on aperture). If you go wider, you lose the ability to really get individual people without being so close there's weird distortion. If you go longer, you end up with not quite enough room.
I used a 40mm lens on APS-C Pentax for a long time, and often found it just a bit too narrow — and sometimes quite a bit. I've got a 23mm for my current (still APS-C) camera, and am finding it really handy for just the situation you describe. You might even want to consider a wide zoom in the 18-35mm range.
A fast aperture is the second factor, but, honestly, at this wide focal length, I wouldn't stress. It's actually harder to use, since you can't isolate a single subject as often; I wouldn't worry about getting faster than f/2.8 — again with experience from using that 40mm f/2.8, that was never the limiting factor. Today's cameras can go up to ISO 6400 with decent results.
It'd be nice to have something faster than the typical f/3.5-f/5.6 of a kit lens, though, because that's a lot darker than f/2.8. But, you might consider not overspending on a lens here and instead rethinking the ambient light aspect — in most situations, a wirelessly-triggered flash will do so much more for your indoor photography than a new lens — see Prime lens or flash: which upgrade will most improve baby photos? for more.