Developing a community and harvesting value from the community are two different challenges.
It's possible to develop a very successful community that struggles to generate meaningful value to the organization. Just ask Reddit.
Organizations develop communities for one of these benefits. Some benefits they achieve quite naturally. Members that actively participate, build relationships, and develop a sense of community are prone to sharing knowledge, purchasing more of the product/service, and increase customer retention.
Usually, however, you need a plan to harvest value without upsetting members. Advertising, spam, and promotional activities usually upsets members. The key is to add value and be trustworthy.
1) Reach out to members with problems. If a member mentions a problem, ensure this is logged into a CRM system (salesforce integrates well with most platforms), and have someone from the organization personally contact them to help resolve the issue. If this is an existing customer, you will probably retain the customer. If it's not, you can make a recommendation or free trial of a product/service you sell that might resolve the problem. This works best when it is a community for the topic, not your products.
2) Develop new products/services for community members. If you can sell community membership, host events (with sponsors), merchandise, community yearbooks, or develop exclusive just for members, you create entirely new revenue streams.
3) Exclusive offers for top members. If you let top members purchase products before others, purchase branded/limited editions of the product, share discounts with friends/colleagues, or even get exclusive information first – they're more likely to keep being a top member (and other members will attempt to reach their level).
4) Sell the community outputs. The community is generated a huge amount of knowledge and fascinating material. Other people might pay money for this material. If you create guides, summaries, or ebooks containing the best material, other people in the topic may buy these.
Don't be shy about harvesting value from the community. If you do it in a sincere, honest, and in a manner that adds value, it should benefit your members and your organization.