One of the women in our homeschooling community has prosopagnosia. It definitely isn't the same as when you don't pay attention to people. She has to work really hard to recognise people using little cues and trick based on their hair and clothing style etc. I'm so glad she explained the condition so that I can be sensitive and make sure I say hello directly and if she looks puzzled I can tell her who I am. I can only imagine how overwhelming large groups must be!