I've not needed this, but it's a nice idea. Do you do this with a combination of sudo/PAM|pubkey auth? I can google, but can you push me off in the right direct? Thanks!
I've not been directly involved, so your googling may well be as good as mine; on a quick look you might have to do this manually using ssss (and then each person encrypts their piece with gpg --symmetric or the like).