Volunteer playtesting is usually just a few hours of watching you over your shoulder (or at the really fancy places, through cameras and one way mirrors) to see what you do when you're exposed to something you haven't seen before so they can see where frustrations lie or what you're inclined to do given a certain situation. They might ask you to fill out a questionnaire afterwards, but it isn't as useful as just watching you go through the game. Or at least that's what it should be. Related question from the opposite angle: How do you get useful data from playtesters?How do you get useful data from playtesters?
At the larger companies you probably won't be meeting anybody that has any say in hiring or any hand in development; they usually have dedicated focus testing people taking notes. Smaller companies have the designers and leads there to watch.
It's like anything else, you might make a contact if you come across as somebody who knows their stuff and aren't a complete idiot. But don't go into a company and say "I have a great idea for a game" because they're not going to listen to you.
