Personal knowledge
In the old days, people lived in smaller communities and really knew each other. These days, its common to see someone regularly walking down the street, or in the hallway, and think you know them.
You need to have met an individual many times in conjunction with others, and have hard their full name from the mouths of others many times to claim that you personally know a signer. If you met the person
only a few times, you will not remember them in court many years later. If you don't know them extremely well, make sure you get a current ID card instead of relying on personal knowledge of the person.