add some more custom config and put every thing in comment in the custom.yaml template
{% extends "email/base_email.txt" %}{% load i18n %}{% block content %}{% if note %}{{ note }}{% endif %}
{% blocktrans with CONF.workspace_name as workspace_name %}You've been invited to join the workspace {{ workspace_name }}.{% endblocktrans %}
{% blocktrans %}Click the following link to complete your registration and access the workspace:{% endblocktrans %}
{{ CONF.site_url }}{{ activate_url }}{% endblock %}