Would it be possible to have GJ2.5 rc so we can test and compare with live?
No, I have not release GJ 2.5 RC as it's incomplete.
Your issue is probably due to Kunenas caching. It caches forum access and doesn't purge it until every few hours. This is reset when a user joins a group though. It seams maybe this isn't happening when adding a user from backend. I don't really know what is happening on your install as no matter what I do I can not duplicate it, but I have an SVN build of Kunena so it's likely a bug in Kunena that is already fixed. Matias nor I can duplicate your problem or see a real reason for it other then the possible caching issue.
I really don't know what else to advise. I've spent over 6 hours creating new installs, testing, and trying to find a cause. I simply can not find one nor can I duplicate it locally and without being able to do so I have nothing to fix.
More feedback will be welcome once I release GJ 2.5 RC. For now 90% of the issues in this thread have already been fixed. I currently can not give you an RC as GJ is mid-development and I have currently half-implementations present in source.