If the presentation is taking place in a conference room, usually walking deeper
into the room will cause an automatic invitation to be displayed. If it does not, you
may want to leave the room completely, by going to the exhibit hall, then go back
to the conference room. If that fails, try clicking on the person that is giving
the presentation.
If the presentation is not in a conference room but in the exhibit hall,
just click on the person that is giving the presentation. You'll be asked
if you wish to join it.
See the complete How To section for more details.