Just add an <else /> to the statement for the JOIN NOW option:
HTML Code:
<if condition="$show['member']">
<td class="vbmenu_control"><a href="login.php?$session[sessionurl]do=logout&logouthash=$bbuserinfo[logouthash]" onclick="return log_out('$vbphrase[sure_you_want_to_log_out]')">$vbphrase[log_out]</a></td>
<else />
YOUR JOIN NOW option (get registration link from navbar?)
</if>