Here is a brief summary. Not my own words, but couldn't have said it better
Business Network International (BNI) is completely legitimate. They are a leads generating operation where each member brings in contacts that other members don't have. They have chapters around the world and in every state in the U.S., but don't cover every city.
They operate as independent chapters, but under strict regulations. The weekly meetings usually include a meal, but the costs can vary greatly with one-time registration fee, annuals dues, and variable costs for local meetings.
Membership is limited to one person per business category, with members encouraged to not only do business with each other when possible, but also to bring leads and referrals to at least one member each week.
The value to you will have a lot to do with the other members as well as total referral activity within the group. You are allowed to visited every chapter in your market - twice each - before joining, so it make sense to visit everyone that has an opening for you - within a reasonable distance.
Members are required to attend regularly and only two absences are allowed each quarter, unless you send another person to attend in your place.
Depending on your products and/or services, membership could pay for itself from just one new customer, or might be a waste of your time/capital.
The regimentation can be a bit overbearing (bordering on childish to some), but they do get results in most cases.
Check out your chosen group well before you join. My chapter losts its charter within 6-months of my joining - With no possible refund on annual dues. I'm told I can join another chapter free (where they have a vacancy for me), but the nearest chapters are 90 to 150 miles distant - hardly an option when weekly meetings are required.
A well hidden secret behind BNI is that they operate as franchises, with chapters getting their support from the franchise owner, which can be an entire state or region within your state. The chapters operate as a profit generator for the franchise owner.