First chmod 770 /your/share/folder
then in smb.conf do something like:
[Public]
comment = Public Directory
path = /home/public
valid users = @public-group
force group = public-group
read only = No
create mask = 0770
force create mode = 0770
directory mask = 0770
force directory mode = 0770
The result is when a user creates a file in the Public share the user:group is useraccount
ublic-group and the permissions are -rwxrwx---