Hi. It seem that groupoffice-com-6.2.3.tar.gz did not include these few binaries based what the files I download at SourceForge.
Anyway I manage to fix the "tar.exe" error, whether or not it actually work I do not know. But here how I did it.
In the .zip files there is an tar.exe
I copy that into \controls\win32
And open Group Office's config.php and added this
Refresh my groupoffice/install and Tar error is gone and I get "OK"