I installed Ubuntu MATE 15.04 in a virtual machine under VBox 5.0.6 on OS X 10.11.
I am having trouble accessing shares, as I do not have permission. The shared folder is owned by root and group vboxsf.
I tried adding my user to the group vboxsf without success using usermod. (I can’t find a command to add a logged in user to a group).
I also tried Manage Groups
from the System Administration menu, but this will not accept my password, There is AFAIK no root
password, only the password I used to create the one and only user.
EDIT ____________
I set the root password (as suggested below) and fixed the issue.
The real problem seems to be that the one and only user was created with custom permissions, rather than being an administrator. I fixed this as root.