Can a directory belong to more than one group?
If your filesystem supports posix acl extensions, then yes you can have more than one group. Most normal unix/linux tools and programs wont show you more than the default user:group:other stuff.
Use getfacl <file/dir>
to get current acl settings and setfacl
to set them.
$ getfacl .
# file: .
# owner: root
# group: src
user::rwx
group::r-x
mask::rwx
other::---
# only user root got write access and src group read/exec access
$setfacl -m g:root:rwx .
$setfacl -m g:staff:rwx .
# and now root and staff groups have full write access
$ getfacl .
# file: .
# owner: root
# group: src
user::rwx
group::r-x
group:root:rwx
group:staff:rwx
mask::rwx
other::---
See man mount
for mount options
ext2:
acl|noacl
Support POSIX Access Control Lists (or not).
See man getfacl
and man setfacl
for acl examples and options.