Choose a schedule and stick to it. Anyone who's been on a never-ending email thread knows that getting a group of adults together is basically impossible. I've been in more than one book group that died because we just couldn't pick a time to meet. Instead of trying to find a date that all members can attend each month, set a regular schedule and stick to it. Maybe it's the fourth Sunday of the month for book group brunch, or maybe it's every six weeks on a Tuesday evening - whatever your group chooses, put the dates on the calendar and don't change them, even if some folks have to miss a meeting. A regular schedule also makes it easier for members to plan for book group, so you'll get better attendance (and fewer emails!)
Don't be democratic. This might be controversial, but my most rewarding book groups have been those where members take turns choosing the books, rather than trying to choose by consensus. Letting individual members choose titles leads to interesting diversity that can be erased by looking for books that everyone will like. I've been surprised and delighted by book group reads like Breasts and A Brief History of Seven Killings that I never would've picked up otherwise. Plus, books that members disagree on often make for the best discussion. Which leads me to my next tip...
Agree to disagree. When everyone likes the book, the discussion can be dull; different opinions lead to great discussion. But different opinions can also lead to heated discussion, so set the tone early on. Encourage your members to disagree - and encourage them to focus on the book, not the other people in the group.
Do your homework. Read the book, obviously (or, if your "book group" is more of a wine group, make sure everyone knows so you don't have one eager member disappointed to find out that no one got beyond the second chapter). Take some time to check out supplementary material like reviews and author interviews, too. A little Googling or a stop at your local library can turn up wonderful gems. When my book group read Exit West last year, some shared an annotation of the first page that the author did for PBS. It was a quick read, but it gave us new insight into the book and fueled our discussion.
Meet someone new. Whether it's the friend of a friend who's already in the group, or a whole room of new people, book groups are a great way to connect with your community. If you're looking for a book group to join, check out the many, many groups hosted by KCLS in libraries all over the county!