A leads group is quite simply a group of business owners or professionals who share referrals with one another. If I know someone who needs a service that a person in my group covers, then I refer that someone to my group's member. The only requirements needed to join the group are your time and referrals. The benefits are referrals in return and the chance to make some new friends!
One other thing to keep in mind. You must offer an exceptionally good product or service. And you must act quickly on referrals you receive. As a group we need to be able to trust that the referrals we are sending are being taken care of quickly and in the best way possible. You certainly wouldn't refer a friend or associate to someone you didn't feel would take the very best care of them!