Samba: Read only for guests with a write list

Is there any way of allowing a read only guest access to a share and also have a write list for allowing some users to modify its contents?

If possible, I wouldn't like to use files and directories permissions.

It would be something like:

[share]
    path = /some/path
    valid users = @somegroup guest
    read list = @somegroup guest
    write list = @somegroup

Anyone can read, but only @group can write:

[Some Share]
    path = /path/to/some/share
    guest ok = yes
    writeable = no
    write list = @group

Only @group can read and write:

[Some Other Share]
    path = /path/to/some/other/share
    guest ok = no
    writeable = no
    write list = @group
    valid users = @group