One of the smartest people I know is very reserved. When he's in a new situation, he just keeps his mouth shut. This may go on for weeks...by the time he finally speaks, everyone thinks he's a flippin' genius.
First, he's always dressed nice-- button down shirt, nice jeans or khaki's and a jacket he can take off if he's overdressed.
He sits near the middle/back (not the very back) and moves a bit forward with each meeting but never to the front.
If someone introduces themself, he shakes their hand, looks them in the eye and answers with first and last name, "John Brown." If they ask questions, he says, "I'm just here to learn."
He immediately turns the focus to the other person, asking the same questions to them, "How about you? What type of business are you in?"
He *listens*-- he's learning the vocabulary, getting to know the other people and learning the dynamics of the group.
He keeps this up until he's ready. This guy understands his natural reserve and his own nature and he's given himself permission to wait until he's comfortable before moving forward.
He's very successful-- probably much more so than the guy that comes on like a used car salesman 8)