Hello Doug,

first in your .py file

_inherit = ['mail.thread', 'mail.activity.mixin']

after in xml file after sheet tag complete,

<div class="oe_chatter">

<field name="message_follower_ids" widget="mail_followers" groups="base.group_user"/>

<field name="activity_ids" widget="mail_activity"/>

<field name="message_ids" widget="mail_thread"/>

</div>

you can also take reference from Odoo standard module.

Thanks