You need to set the directory permissions to allow users to write to that directory. Samba then has a tool for defining the default permissions of files written to that share
Place that in the smb.conf file under the share in question. Keep in mind, that if a user creates a folder in the share, then the default permissions of the folder will be read-only.
The above example will give the owner, members of the owner's group and all other users read-only permissions on any file written to that share.