Or HaChaim on Numbers 1:18/אור החיים על במדבר א׳:י״ח

1 ואת כל העדה…באחד לחדש, and the entire community, on the first of the second month. The Torah announced that the Israelites assembled on the very day the instructions were issued. This is proof that everyone was equipped with a "book" showing who he was and who his father was. The people did not have to go and look for corroboration of their family status by looking for witnesses, etc. This is what the Torah meant by the word ויתילדו being written next to the words "on the first of the second month."

2ויתילדו, "they declared their pedigrees, etc." Rashi comments: "they each brought a book showing their pedigrees and witnesses to their birth in order to prove which tribe they belonged to." Nachmanides does not agree that there was a need for documentation of their status but claims that when each Israelite brought his half shekel he announced who he was and to which tribe he belonged. Moses placed the shekalim of each tribe in a separate box so that he knew how many members each tribe numbered. Perhaps the reason that at this point it had become necessary to produce documentation plus witnesses was to foil the attempt of any bastard who was aware of who his father was and who would produce a book listing his father's prior pedigree claiming to be a member of that tribe based on information provided by his mother.