social/mail_activity_team/views/res_users_views.xml

<?xml version="1.0"?>
<odoo>
    <!-- Update user form !-->
    <record id="view_users_form_activity_teams" model="ir.ui.view">
        <field name="name">res.users.form.activity.team</field>
        <field name="model">res.users</field>
        <field name="inherit_id" ref="base.view_users_form"/>
        <field name="arch" type="xml">
            <data>
                <field name="signature" position="before">
                    <field name="activity_team_ids" widget="many2many_tags"/>
                </field>
            </data>
        </field>
    </record>
</odoo>