hr_employee_views.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <!-- Show portal users in employees -->
        <record id="view_employee_form" model="ir.ui.view">
            <field name="name">view.employee.form</field>
            <field name="model">hr.employee</field>
            <field name="inherit_id" ref="hr.view_employee_form"/>
            <field name="arch" type="xml">
                <xpath expr="//group[@name='active_group']/field[@name='user_id']" position="attributes">
                    <attribute name="domain">[]</attribute>
                </xpath>
            </field>
        </record>
    </data>
</odoo>