Learn the Lay of the Land - Don't jump right in and start posting until you get a feel for what kind of people the other members are, what their conversational style is, etc. More than anything, you're trying to find out if this is a place you want to network before you make your presence known, because you actually damage your reputation if you introduce yourself and then disappear. Better to never say anything and quietly slip away if it's not a place you're going to stay.
The Power of "Hello" - Always start with a personal introduction before you join in the conversation. This gives context to what you say, a critical aspect to good communication. A good introduction should be upbeat, personal, tell a short story, tell briefly about your business without being a sales pitch, invite people to connect with you, and affirm your commitment to participate in the group.
Dive In, But Don't Splash - Once you introduce yourself, join immediately into the conversation. You should have already gotten your "lurking" over with before you introduced yourself, so there's no reason to delay. However, don't be a boor and hog the conversation. Generally, a good guideline is to be in about 2-3 conversations at a time. Also, rather than replying to every single person involved in the conversation with short responses, better to post just once every day or two with a more thoughtful, reflective response that addresses multiple people's posts. This is the online equivalent of being a good listener, rather than listening just being a matter of waiting for your turn to talk. You'll establish far more credibility this way as being thoughtful and knowledgeable - in other words, an expert.