Piece of identification needed at first registration day of a US conference

If you're registered as a student (or some other role that gives you a discount) then you may need to bring proof of that, but otherwise I've never attended a conference and had to bring anything in order to register.

If the organisers of the conference expect you to bring something then they'd almost certainly tell you. Check the information you've been given, and if they say nothing about bringing proof of ID then you don't need it.