source: branches/samba-3.5.x/docs-xml/smbdotconf/security/forceuser.xml

<samba:parameter name="force user"
                                 type="string"
                 context="S"
                 xmlns:samba="http://www.samba.org/samba/DTD/samba-doc">
<description>
    <para>This specifies a UNIX user name that will be 
    assigned as the default user for all users connecting to this service. 
    This is useful for sharing files. You should also use it carefully 
    as using it incorrectly can cause security problems.</para>

    <para>This user name only gets used once a connection is established. 
    Thus clients still need to connect as a valid user and supply a 
    valid password. Once connected, all file operations will be performed 
    as the &quot;forced user&quot;, no matter what username the client connected 
    as.  This can be very useful.</para>

    <para>In Samba 2.0.5 and above this parameter also causes the 
    primary group of the forced user to be used as the primary group 
    for all file activity. Prior to 2.0.5 the primary group was left 
    as the primary group of the connecting user (this was a bug).</para>

</description>

<related>force group</related>
<value type="default"/>
<value type="example">auser</value>
</samba:parameter>