You will need to get your hands to command line and use "chown",
If it's like this:
+ you have a "info", "sales" ... with users:
which need to create/ modify the files in their "shared" directory ?
How to add users to group?
You need to create that shared directory and allow them doing things they need.
chmod 770 /home/info_shared
chown -vR nobody:info /home/info_shared
this will create "info_shared" directory and allow users in group "info
" to write.
How will you share the directory it's a more complicated way,
you can do it with Samba
or if the users will be using the "server" to login, they will be able to browse the directories on the "server" (in this case you are done).