My experience was that they only cared about the number of meals you had loaded on the card. I was a party of 4, and I ordered all sorts of variations with my card. Two meals for the kids, and then two adult meals at a different venue for the wife and I while the kids had a sitter. Six counter services at one time...or 1 or 3, or whatever....So even less important than head count, you just had to have enough credits left. I got the impression at some restaurants that I could have had a big party and used my whole meal plan for the week at one meal for 28 guests, if that is the route I wanted to go.
I think your bigger problem would be if you "declare" the new guest, as Disney wants everyone on the same plan.
To answer the original question, I would say with 95+% certainty they will not check id's at the restaurants. So if you book for 4, and you show up with a different crew, the only one who MAY matter is the one who gives the card.
I subscribe to the philosophy that if you act like nothing is out of place, then so will your servers.
One other note, they will accept an adult credit for a child meal, but not the reverse, of course.