Add yourself to the vboxsf group within the guest VM.
Solution 1
Run sudo adduser $USER vboxsf from terminal.
(On Suse it's sudo usermod --append --groups vboxsf $USER)
To take effect you should log out and then log in, or you may need to reboot.
Solution 2
Edit the file /etc/group (you will need root privileges). Look for the line vboxsf:x:999 and add at the end :yourusername -- use this solution if you don't have sudo.
To take effect you should log out and then log in, or you may need to reboot.