I have a similar use case with independent local groups under an umbrella (national) organization. Much of the documentation (training, standards, policy, education) and downloads (template documents) are global but day to day operations are almost exclusively local. Our intention is to encourage cooperation and discussion between all groups so a key driver is to have all users able to participate globally.
If I had multiple instances of Discourse then I’d very quickly discover that category management is nowhere near as important as user management. Each post has an owner and it is generally important to retain that relationship particularly if I want global topics to flow naturally from local topics.
That’s why my prototype used one Discourse instance with each group owning its own category. The only reason that I might not consider doing it this way is if the number of local groups exceeded Discourse’s capacity to process that many categories. I only have 200 local groups in my country and even worldwide there are only 400 so this is not a problem for me.
There are several cost benefits from using one Discourse instance:
- not paying for all those local instances
- centralizing Discourse expertise and standardizing processes, e.g. making it easy to add any new groups
- emphasizing that all local groups are part of the global group, e.g. making it easy for users to transfer to another group