VirtualBox - Sharing a folder
There are no guest additions for macOS in VirtualBox. The shared folder feature of VirtualBox won't work here.

You have to configure a share on the host system and access it from inside the guest with one of the common methods:


Finder > Go > Connect to server... cmdK
Terminal: mount -t smbfs //username:password@server/share .../mountpoint
Finder window > sidebar > Shared > choose share > 


Some network connection between host and guest is required (e.g NAT or bridged adapter). Using the latter requires to set up an IP-address in macOS in the same network as the host's adapter.



To set up a bridged adapter for a guest (which has to be shut down) do the following:


Choose the VM > Settings > Network > Adapter 1 > Check Enable Network Adapter > Attached to > Bridged Adapter > choose a network interface of the host (usually either Ethernet or Wi-Fi)
Check the IP config of this interface in the host (e.g. IP= 192.168.0.2 Netmask= 255.255.255.0 Gateway=192.168.0.1 DNS=8.8.8.8)
Use the same config in the guest but choose a different IP (e.g. 192.168.0.111)
Connect from the guest to the host with cmdK > enter smb://192.168.0.2/Share_name > enter the credentials