To be a member you have to pay an annual fee between $24.99 (BackPackingLight) to $1,495 (The CR). Multiply this by a few hundred, or a few thousand members, and you have a business model.
This business model allows you to hire a community manager (we hired two for CommunityGeek) to cater very closely to the needs of members. It also allows you to create lots of perks/benefits for members, host member events etc...
The value here isn't in the content you create, although great content helps people sell it to their boss. The value comes in several forms.
1) The ideas shared within the community that can save or earn you thousands. If a conference is worth $x,xxx to attend, an ongoing community should easily match that. If you get a great idea from a community that increases retention rates or conversion rates by 15%, how much is that worth over the lifespan of the community?
2) The feeling that you're one of the best, elite, within the sector.
3) Deals with partners/vendors to get discounts on their products/services.
4) Getting immediate help with any question/problem you have from the experts in the field.
These models work best when they're exclusive. They work best when you only accept the best. That means deliberately earning less (at least in the short term).
It also makes you independent of advertisers. This is a bigger benefit than we might think. You can stop the attempt to get everybody to join the community. Instead you can focus on getting exactly the right people to join.