It seems that there is a Linux user (on an Ubuntu 12.04.4 server) who is unable to access a shared directory, even though that user belongs to the group which owns the directory. Here is an example session:
$ whoami
username
$ cd /shared_directory
bash: cd: /shared_directory: Permission denied
ls /directory
ls: cannot open directory /shared_directory: Permission denied
$ ls -ld /shared_directory
drwxrws---+ 116 root groupname 4096 Jun 11 11:35 /shared_directory
$ getent group groupname
groupname:*:username:otheruser
sudo adduser username groupname
The user `username' is already a member of `groupname'
It also appears to be the case that this user is the only group member to be experiencing this permissions issue.