有个解释参考下：“We found the secret to accessing shared folders in an Ubuntu 11.04 virtual machine in VirtualBox.VirtualBox adds a user group called “vboxsf” to the Ubuntu 11.04 virtual machine. Any user that needs to have access to shared folders defined for that virtual machine need to be a member of that group.”
解决办法：
1、首先进行Guest Additions安装，Devices->Insert Guest Additions CD-image，自动播放功能安装会提示安装，安装成功以后，通过/etc/groups能查到名为vboxsf的用户组。（请参考http://askubuntu.com/questions/161759/how-to-access-a-shared-folder-in-virtualbox里对于“First, please make sure you have installed the Guest Additions”的描述）
2、根据问题原因链接里的办法，（在ubuntu16.04里没由找到跟链接一样的图像操作界面，因此）在命令行使用usermod -a -G vboxsf (user)将用户添加到vxboxsf用户组里。
3、问题解决后，在/mnt目录下就能看到挂载成功的共享目录。