ACP > Look / Feel > Skin Manager > (choose skin) > Manage Templates / CSS > Global Templates > UserInfoPane
Find:
<li class='post_count desc lighter'>
{parse expression="$this->registry->getClass('class_localization')->formatNumber( intval( $author['posts'] ) )"} {$this->lang->words['m_posts']}
</li>
</if>
</ul>
Add this after the </if> tag but before the </ul> tag
<span class='group_title'>{$this->lang->words['m_joined']}</span> <span class='post_count desc lighter'>{parse date="$author['joined']" format="joined"}</span>
Should look like this
<li class='post_count desc lighter'>
{parse expression="$this->registry->getClass('class_localization')->formatNumber( intval( $author['posts'] ) )"} {$this->lang->words['m_posts']}
</li>
</if>
<span class='group_title'>{$this->lang->words['m_joined']}</span> <span class='post_count desc lighter'>{parse date="$author['joined']" format="joined"}</span>
</ul>
Then you can save it. If you have any questions/problems, post in the topic.