No no, this is Rob Goldstone, the middleman, asking Don Jr for the names of the people in Don Jr's team, "for security".
For a meeting in Trump Towers!
It's explicit in the email chain.
It would have made sense if Don Jr had been asking the names of the people arriving at his family offices, at the very least so the building security folk could admit them. But no. Indeed Don Jr claimed he didn't know the name of the person(s) coming to the meeting. Errr...