I have noticed that if an AD user has a PrimaryGroup different than the default CN=Domain Users that they don't sync to O365 portal. When I change it back and initiate a sync cycle, seems to work.
Now I want to discover all of these in each of our domains. I don't think this is in the Global Catalog so was trying to wrestle a one liner to find them, domain by domain. I get about a dozen users back but then it errors and not sure what it means.