I believe one significant issue is that, like so many things in Linuxland, the same command line solution isn't universal i.e. there are alternative methods recommended/required in by some Distros including:
- sudo adduser [username] vboxsf
OR
- sudo usermod -a -G vboxsf 'whoami' [which self identifies the user identity]
There may well be others.
There are also some Distros that have a GUI app for managing Usergroup membership; and in more secure environments where users do not have full admin/sudo privileges, e.g. a standard Debian installation, the commands can only be used by someone signing in as 'root,' or by first granting the user full root/sudo privileges.
Some Distros, e.g. openSUSE, also require a further step to grant User access to the shared folder location e.g. sudo chown -R `whoami`:users /media/
All of these anomalies make it very difficult for the Manual maintainers to provide/maintain a detailed procedural guide that can reliably cover all options.