Linux facl minihowto

First step is

apt-get install acl

Allowing members of other groups the full access to a directory, resursively:

setfacl -m d:g:groupname:rwx -R path/foldername

d means default so modifying the default ACL results that all of newly created files and directories will inherit this setting.
Modify the permissions of existing files and directories only [not the default]

setfacl -m g:groupname:rwx foldername

Important notes regarding files: Files can't have default ACL because they can't have child objects. An access ACL for an individual file can override the default: if a file has a special ACL that conflicts the inherited ACL, the file ACL wins: owerwrites the inherited default one.
Clearing an ACL, e.g.:

setfacl -x u:johnny /path/folder