Access control lists
The straight answer is access control lists (ACLs). Yeah, you can find a counterexample, but they're good enough in practice (unlike mere group writability which requires that users think about it all the time). What they do require is that the system administrator (root) define the groups, if you want files to be shared only by a named group (root can choose to delegate, for example by accepting groups from LDAP, but that's another story).
You do need participating users to have a umask of 022. If they create non-world-readable files routinely, this scheme won't work. But if they have a restrictive umask, it's presumably because they don't want to share files anyway.
Ubuntu doesn't enable ACLs by default, so there's a one-time admin requirement. Edit
/etc/fstab using your favorite editor, and change every line corresponding to a filesystem where you want to share files: add
acl to the options. (Make sure not to change any other line, and not to use an editor that wraps long lines.) Here's an example line with the
acl option added:
UUID=5e1ec7ed-face-dead-beef-c011ec7ab1e5 / ext4 errors=remount-ro,acl 0 1
For the option to take effect the first time, use a command like the following (for each filesystem):
sudo mount -o remount,acl /
Install the ACL tools from the
Setting up the shared directory
To have files shared by the group
setfacl -m group:mygroup:rwx /path/to/shared/root
setfacl -d -m group:mygroup:rwx /path/to/shared/root
If people create files and copy them to the shared directory, the files will be world-readable (because of the umask) and anyone in the group can add and remove files (because the group is group-writable). People can't edit each others' files, but that's a good thing or you'd run into editing conflicts straight away.
If you don't have a unix group, you can add users one by one:
setfacl -m user:bob:rwx /path/to/shared/root
setfacl -d -m user:bob:rwx /path/to/shared/root
If you do want people to be able to edit files in place, you also need something to prevent editing conflicts. That's version control.
You don't need any of this to share a git repository. You know there are solutions like gitosis, so use them.