Linux – VirtualBox: mount a shared folder automatically in Gentoo

I'm currently writing a template (in veewee) to build a vagrant box running Funtoo x86_64.
Currently, my box fails the last step for vagrant basebox validate, in which a shared folder has to be mounted automatically in the guest system.

What are the prerequisites for automounting of shared folders? I searched for it in the documentation for Gentoo and for VirtualBox, had a look at templates for Ubunto boxes – but still don't know how to replicate the automounting behavior of the default Ubuntu box.

I'm using genkernel with the gentoo-sources. The virtualbox-guest-additions are installed and the virtualbox daemon is started.

I'd love an answer containing these requirements (with some of my best guesses):

• Kernel config (NFS v3/4, automounter, …?)
• Userspace programs (do I need additional packages? which daemons in which runlevel?)
• Directories and permissions (do I need a /media dir belonging to some user, do certain users like vagrant and root need a membership in the vboxguest group or a vboxsf group – which does not exist yet?)

but any kind of help would be great – even a single link or a hunch…

UPDATE: What I did so far…
I can mount shared folders manually, what I'm interested in is automatic mounting.

The VirtualBox website mentions a group vboxsf that should be created on installation of the additions – I don't see that group in /etc/group, I have vboxguest, though.

It also says that shared folders are mounted under /media with a prefix of sf_ (controllable by /VirtualBox/GuestAdd/SharedFolders/MountDir and /VirtualBox/GuestAdd/SharedFolders/MountPrefix). I don't have a /media directory either – and both properties were not set on my VM.

I created both the directory and the group and set the properties to their default values, but I'm still out of luck.