help for a new samba user

[gdbnet]

hello, i'm an italian student (i'm sorry for my english).
i'm a new samba user and of course i have a little (i hope) problem.
i'd like to configure smb.conf to do this 3 thinghs
i'd like to have 3 dir shared via samba with this option
the first dir shared by all user
the second one shared (and visible) only by users of xxx group
and the third one read-only by all user and writeable by only the yyy user

i tried do do that but :
1- is it necessary that the group-owner of the second dir is the same that
can acces into via samba??
2 - is it possible to make this dir visible only to the users of the group
that share this dir ?? (i was not able to do that)

thanks a lot.