It's hard to find two organizations that developed a community the same way.
Some begin with big top-down processes driven by the CEO. The CEO, or someone at the executive level, clears the path, provides the resources, and pushes the process through (often against skeptical employees).
Some begin bottom-up. A few employees begin a community with a small goal. The community is successful. They gradually expand their efforts. They bring more employees in to the community. Over time they attract the attention of an exec.
Others begin at the middle management level and are a cross between the two.
In the 120+ organizations we've worked with, there are few common structures between them. Beyond having a full-time community manager, what works in one organization usually won't work in yours. The culture, complexity, personalities, and charisma of the community manager vary too greatly.
What does matter, more than official team structure, is having a ragtag band of believers. A group of people involved in developing the community (often unofficially) and help make things happen. A community manager, alone, usually can't get the job done. The challenge is to build that unofficial band of believers to keep pushing the project.
Even in CEO-directed projects, you need to build your own believers in the project to make it a success. Forget the internal structure. Reach out to the people likely to believe in the project. These will be people who either know about the power of communities, have a good relationship with you, or have the personality to try new things.