I will soon take part in my first congress in the USA.
What piece of identification is needed to register on the first day?
I don't want to carry my passport with me all the day of the conference so is a driver licence sufficient?
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.
I have attended a few conferences or workshops held inside (US) government owned facilities, and for those I had to show ID at the gate. In the case of the site where highly classified work is done my foreign colleagues had to provide extra documentation before coming and we all I had to promise to stick to certain parts of the facility and to return our temporary badges before leaving.
That's the only circumstance in which I've been asked for ID on showing up for a conference.