You can 'man' any command that you know (well most anyway) but the man pages ARE confusing when you are fisrt starting out, so with this in mind I'll outline a few things to help you on your way...
first thing is to set the group access to the files... lets work with /usr/local/share as an example. Set the group users as the group for this directory,
chgrp users /usr/local/share
Thats pretty easy. Now you want to allow folks in the group users (and the owner) to have full permissions to access the folder but noone else...
chmod 770 /usr/local/share
you may want users to only read and execute things in this directory (owner has full access...
chmod 750 /usr/local/share
basically the easiest way to set file permissions is using the chmod command with binary input.
so 750 gives the permissions drwxr-x--- to the above dir.
0 gives --- 1 gives --x 2 gives -w- 3 gives -wx 4 gives r--
5 gives r-x 6 gives rw- 7 gives rwx
and the location of the values is also important...
chmod 750 here the 7 is set for the owner of the file or dir. the 5 sets r-x for the group, and 0 is set for the oranisation or anyone who can access the filesystem.
Thats it in a nutshell, there is alot more to chmod but this will get you well under way.