I started a book group through the Meet Up website. It's quite expensive, but I invite a voluntary donation of £1 to cover running costs.
After having been in a book club which was taken over by a lady with a very didactic approach, I was determined to make mine much more relaxed! Our unwritten rules are:
We meet in a pub and the description on meet up stresses that it is informal and relaxed.
Everyone is welcome, regardless of whether or not they have read the book, as long as they don't mind spoilers.
We all contribute anything we fancy reading to a list of suggested reads. Every month, we select one at random. Anyone has the option of vetoing any book if they don't fancy it (we ask for vetoes before we make our random selection).
I take along a list of questions on the book being discussed, but we often don't need them. It's really just an informal chat which often branches into other areas!
The advantage of Meetup is that we have a central core of about six members, with up to ten extras who come and go. There's a really eclectic mix of ages and interests which is great. The group's been running for two years now and we have formed friendships and branched into separate activities (sports, meals out, parties,etc).
Good luck with starting a group!