Hi,
As samba usernames are usually independent of linux-usernames it seems logical to me that samba does not use the samba usernames to access files and directories.

So if a directory is shared, who should be allowed to read/write there so that all samba users may access this directory? (Should not be everyone i believe...)

If you make a local group called something like sambausers. Then add the local users that you want to access that directory/file and give that group read/write permissions. Then set the appropriate permissions in samba too. The way I understand it is that samba both checks local and samba permissions when accessing a file.

For samba, it requires that you have a UNIX/Linux user for every Samba user you have seeing as the home directory is that of a linux user. So if you have the user 'joe' on your linux box and want him to be able to access a samba share remotely. You need the equivalent Samba user named 'joe'. As far as if you want an entire directory for everyone to write to, just create the directory and make a group and put "valid users = @group" and to your "write list" param put the "@group". Yes, the @ is required because it denotes a group.

Thank you very much for your answers, I'll go and have a try ;-)