None. If you keep a list of drivers you can work out who's joined and left from that on each scoring update.
After spending again some hours - and looking for the failure I'm making - I just did a quick and very simple API function call logger which allows to analyse in a spreadsheet whether functions are being called or not and how frequently they are called. It's really easy so don't expect too much - but was quite annoying typing ;-) Did spend some time to tweak the compiler/linker settings to have an Visual Studio project which really compiles and links Win32 and x64 versions in debug and release mode. There are still two warnings but no issue at all. If you want to use just the DLL's feel free. DLL's and source code available at Git: https://github.com/MerlinCooper/rF2_API_Logger
You could monitor mResultsStream for the join part messages but like Lazza said, you could just track the drivers on your own via UpdateScoring.
