If you wish to question a speaker, you'll need to post a message on the Twitter account of the hall where the report or hands-on lab is held.
Reports and hands-on labs will be held in five different halls, so you should post tweets to one of the five accounts:
@phd_valdai — Valday Hall
@phd_amph — Conference Hall
@phd_prhall — Press Hall
@phd_ural — Ural Hall
@phd_baikal — Baikal Hall
For example, you wish to ask a question to Maxim Goncharov (senior threat analyst at Trend Micro) who is speaking on hacking services underground market in the Conference Hall. Then you just need to go on Twitter, log in to your account and post the question on the right hall's account.
Of course, there will be many people willing to question speakers. However, you have good chances that your question will be asked if it is really interesting.
You can use the hashtag #PHDays on Twitter to communicate with other participants and guests of the forum.