Because the unix permission model sucks, it is very hard to have a shared directory where everybody has full access to each others files. The same is true when making a shared samba archive. Luckily, samba has forcing options.
[archief] comment = save stuff here path = /path guest ok = yes writeable = yes browseable = yes force user = samba force group = samba create mask = 0660 force create mode = 0660 directory mask = 0770 force directory mode = 0770
The key here is the force modes.