My last two jobs were the only ones that had any real type of holiday celebration. Last job was with a law firm, and the bash was very nice, upscale, at a nice place, and was for employees only. As a single person, I actually preferred this. They were, however, very generous about including all temps, and new hires, even those who had not yet started. Which was nice.
My current company has done a number of types of holiday parties; when I started, it was after hours, in the evening, and you could bring a guest. I don't really know what the parameters were, as I had no one to bring, but we are a small, more laid-back place than my last job, so I do know while most brought their SO, many brought an adult child, or parent, but I do believe at least some of the staff knew the guest. I personally wouldn't be comfortable bringing a friend to a company holiday gathering, mainly beacuse they wouldn't know anyone, and would be bored.
Later years, it was later in the day, cocktails etc. and employee only. So you went right from work to the party. Again, having no one to bring, I actually prefer this.
My one boss, however, who is Queen SS, once had a bit of a hissy fit. You see, she wanted to attend one of the "bring a guest" parties, but as a single mom, well, there was no one to watch her 13 year old daughter. So she asked if she coud lbring her as her guest, and was quite put out to be told, sorry, that won't be possible. One of the things about being a parent, is perhaps missing out on something if you don't have anyone to watch your kids or some such thing.
She was not happy, but we were all like, um, who DOES that? Child would have been bored out of her mind, and, as alcohol was served, if she somehow got some, the company would have been liable. but this particular boss doesn't think ANY rules apply to her. ever. She then flounced around making PA comments about how it was horrible daughter couldn't attend, and so she couldn't as well. Quite honestly, she wasn't missed all that much.