Simple, first go up and introduce yourself to the person you wish to learn more from. Have a light conversation about their work and yours. Be sure and be attentive and listen to them. Be sure to explain to them you are wanting to learn more about the subject at hand. Don't bombard them with questions there at the meetup, most folks there have been working all day and are trying to relax a bit. Near the end of your introduction ask them politely if they would mind you emailing them with some SPECIFIC questions. When you do email them, ask SPECIFIC question not general ones. Also be sure to thank them for their time. Being yourself and nice goes a LONG way with most folks.
Good luck! Mike