Read the books

Attend the meetings

Collect book reviews and recommendations for your group's reading list.  Be willing to read books that aren't your type.   

Practice courteous group behavior. 
Be on time. Take turns talking. Listen. Disagree tactfully. Contribute, but don't dominate. Stick to the point. Help draw out quiet members and steer the group back from side trips. 

Be prepared for discussion even if you aren't leading it. 
Do a little research on unfamiliar subjects.  Write down your reactions and questions as you read, and bring your notes to meetings.  Push yourself beyond saying “I liked it” or “I didn't like it.”  Trust your opinions, and support them with passages from the book.

Agree that you can disagree about books.
Try to understand other opinions and viewpoints, especially when you disagree with them.  Don't take different opinions personally.  Don't try to force your group to agree on a “right answer”—fiction (and often nonfiction) is open to interpretation.