Usershares used in Ubuntu built-in Sambe share

The default setting is that all file are read-only, even the file owner
you may change it yourselves in /var/lib/samba/usershare/share_name

path=/path/to/shared/folder
comment=
usershare_acl=SID:R
guest_ok=y

The red letter means

• R - read-only
• F - full access
• D - deny access

just change from R to F for your write access

Ref:
Integrating Ubuntu Hardy Heron 8.04 with Active Directory
HowTo Configure Shares on a Linux File Server in a SOHO LAN