You can choose a more traditional hierarchy or a more decentralized model. Here's an example of each:
In the United States and Canada program, efforts are run by one paid staff member who serves as the Program Manager and makes strategic decisions about the direction of the program, the assignments instructors are encouraged to use, and support resources available for students and instructors. The Program Manager works closely with a group of Regional Ambassadors, who are each responsible for a specific geographic region in the United States and Canada. The Regional Ambassadors work to support Instructors and Campus Ambassadors, providing training and support for them to ensure in-person support for students works well. The Program Manger also coordinates the work of Online Ambassadors, who support students on-wiki. This program currently supports about 75 classes each term.
In the Czech Republic program, efforts are more decentralized. Wikipedia Ambassadors pair with Instructors. Chapter members serve as central resources for the program, but many of the decisions about what direction the assignments should take get made at the class level, rather than the program level. This program currently supports about 6 classes each term.