project-tools/project_assignees/views/project_task.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <record id="view_task_form2_assignees" model="ir.ui.view">
        <field name="name">project.task.form.assignees</field>
        <field name="model">project.task</field>
        <field name="inherit_id" ref="project.view_task_form2" />
        <field name="priority" eval="99" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='user_id']" position="after">
                <field name="assignee_ids" widget="many2many_tags" />
            </xpath>
        </field>
    </record>

    <record id="view_task_search_form_assignees" model="ir.ui.view">
        <field name="name">project.task.search.form.assignees</field>
        <field name="model">project.task</field>
        <field name="inherit_id" ref="project.view_task_search_form" />
        <field name="priority" eval="99" />
        <field name="arch" type="xml">
            <filter name="my_tasks" position="attributes">
                <attribute name="domain">['|', ('user_id', '=', uid), ('assignee_ids', 'in', uid)]</attribute>
            </filter>
        </field>
    </record>
</odoo>