You could call some of them by physical characteristics, (the man with the long hair; the girl with the crooked nose), but if you call all 20 of them either by names or physical traits, the reader will lose their place. I recommend splitting them into groups, so that the POV character can refer to them as “group A” instead of listing a bunch of physical traits. This essentially cuts down your character count, without actually doing so.
As far as introducing names, you can have a few characters say “Hey, I never met you, my name is John Doe” or you can have another character call a person by name, and the POV character thinks, “AH, the man with long hair is Justin!” or whatever. This will work even in a high-action, tense situation where you need to use names to cut down on words and sentence length.
As for personal experience, I went to a retreat where I knew next to nobody. It was a mixture of me asking my travel companions, (who had been there before) “Who is that person, over there, next to the coffee machine, with black hair?” and me making mental notes of people–similar to the physical traits tip I gave you earlier–so that I could tell them apart. Then I just talked to people, and kept a mental log of the people I talked to, running through their names and what they looked like in my head and on paper (Okay, the booklet with their names in wast the paper; I didn’t actually write them down). This is my experience, but you’d probably have to modify it to put it in a book; it’s pretty boring as it is.
Anyway, there’s what I got for you! Hopefully it’s helpful.
Quoth the raven, "Nevermore!"