To set up a shared folder between an Ubuntu-mate host running Virtualbox and an openSUSE guest.
Note: openSUSE serves as an example - this procedure should work with any Linux guest.
BEFORE YOU START
Set up a folder to share between the Ubuntu-Mate host and the openSUSE guest in Caja e.g.
Note: “name-of-your-user” is given as “nilo” in the example that follows.
Install Virtualbox from the Ubuntu-Mate software boutique.
Install an openSUSE Leap 15 virtual machine. Then close it down.
IN THE VIRTUALBOX WINDOW
In the Virtualbox window select the menu “Help” and the item “Virtualbox Website…”
From the Downloads section download the “Virtualbox Guest Additions” and install them directly.
Add the shared folder to the virtual machine using vBox graphical interface. Make sure to select automount and make permanent.
Login to the virtual machine using a root account
Check the vboxsf group exists
~$ grep vboxsf /etc/group
Check the user is not already in vboxsf group
~$ id nilo uid=1000(nilo) gid=1000(nilo) groups=1000(nilo),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),109(lpadmin),124(sambashare)
Add the user nilo to vboxsf group
~$ sudo usermod -a -G vboxsf nilo
Check the user groups again
~$ id nilo uid=1000(nilo) gid=1000(nilo) groups=1000(nilo),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),109(lpadmin),124(sambashare),125(vboxsf)
Reboot and login as nilo
This is reposted here to make it easy to find and for the sake of redundancy (two copies are better than one). Many thanks to the original poster.