Shared folders

Host side

# get UUID for VM using
vboxmanage list runningvms 

# or for stopped VMs
vboxmanage list vms 

# create share
vboxmanage sharedfolder add <VM UUID> --name <share name> --hostpath  </path/to/host/folder> --transient

Guest side

Explicit mount

sudo mount -t vboxsf <share name> </path/to/guest/mountpoint>

Auto-mount

The guest additions includes a service that will do guest-side auto-mounting. You just need to find and start that service. How you do this depends on how the guest additions were installed (from the ISO, or from the guest's package manager), and what guest you are using.