Some already have their own domains but I would set this up independent of them. Probably as subdomains. It would be up to them if they want to redirect to their chapter page or just leave the chapter page as a general info page that points back to their own site. Ideally I would want to provide all the functionality they require so they can just migrate over.
At the top level there will need to be a super admin that can control the parent site, the initial deployment of the chapter sites and has ability to access the chapter sites in case the owner of the chapter site were to step down or become unavailable.
If I could put together a solid set of pages, features/functionality I don't imagine they will differ much. There may be some distinction of chapters based on their focus. for example a set of chapters might focus on topic A and another on topic B. I guess, in that case, I should say their will be groupings of chapters. All chapters will have a large percentage of pages and features/functionality in common. Each chapter grouping may require extra pages and features/functionality but that will also be common with the other chapters in the grouping.
Hope this helps clarify things a bit. I'm just getting started with this project so there are a number of things I haven't worked through yet. This is largely a discovery process at the moment to determine which platform is going to work best for this project. ProcessWire seems super flexible and could be perfect for this task especially since the community I am putting together does include a good size percentage of developers. The may appreciate an open source, developer friendly CMS with a solid API over Wordpress