partner-contact/portal_partner_select_all/wizard/portal_wizard.xml

<?xml version="1.0" encoding="utf-8"?>
<flectra>

    <record model="ir.ui.view" id="portal_wizard">
        <field name="model">portal.wizard</field>
        <field name="inherit_id" ref="portal.wizard_view"/>
        <field name="arch" type="xml">
            <field name="user_ids" position="before">
                <group>
                    <field name="set_all_users" string="Invite all the contacts"/>
                </group>
                <div class="card bg-warning mb8" attrs="{'invisible': [('set_all_users', '=', False)]}">
                    <div class="card-body bg-warning">
                        <i class="fa fa-warning"/> You're about to invite to portal all the contacts list below.
                    </div>
                </div>
            </field>
        </field>
    </record>

</flectra>