Localization tools will strip any newlines in `trans` blocks, thus this block will be send as a single line.

To keep the formating, break this into multiple blocks like this
```
{{ _('Hi there') }},

{% trans sender=sender %}
{{ sender }} has invited you to join mozillians.org,
the community directory for Mozilla contributors. You
can create a community profile for yourself and search
for other contributors to learn more about them or get
in touch.
{% endtrans %}
```