A newly established small business networking group has it's own energy and "esprit de corps" Founding members feel a part of building something bigger than themselves and are out to prove it. Being among the first to join also offers a much more likely chance of locking up your business category exclusively before anyone else claims it. That has huge implications for future referral business.
An established small business networking group enjoys the pluses that accompany strength in numbers. With a sufficient roster of members, it's easier to recruit and presents exponentially more opportunities for referral business to come your way. The experienced members already know the drill and can then be very helpful in putting the new members at ease.