This doesn't need to be listed directly, just in the pulldown menu that displays their rank and post count, and other such info. If the info is already in the array of info that populates that menu, I can add this myself.
The information will be in
$message['member']['registered'] // like July 12, 2015, 01:04:23 pm
$message['member']['registered_timestamp'] // like 1436706263
You can use those in the dropdown or in the poster info area, that is done in GenericMessages.template.php in function template_build_poster_div
So decide where you want them, and add them using one of the above, you can use the timestamp to create your own basic date instead of the date / time.
I decided to use:
strftime('%d %B, %Y', forum_time(true, $message['member']['registered_timestamp']))