Molok, if you mean where I think you mean, you could try putting this bit of code in the usercp template. Admin CP => Templates => Modify => User Control Panel Templates [Expand] => usercp. Find:
$privatemessages
Below that put the following code (before $forumlist):
PHP Code:
<table cellpadding="0" width="100%" bgcolor="#0A293E" cellspacing="0" border="0" align="center"><tr><td>
<table cellspacing="1" cellpadding="4" border="0" width="100%">
<tr id="cat">
<td bgcolor="#336D95" background="https://vborg.vbsupport.ru/images/catagory_backing.gif" background="https://vborg.vbsupport.ru/images/catimage.gif" colspan="5">
<table cellpadding="0" cellspacing="0" border="0" width="100%">
<tr id="cat">
<td><a href="invite.php?s=$session[sessionhash]"><normalfont color="#000000"><b>Invite a Friend</b></normalfont></a></td>
</tr>
</table>
</td>
</tr>
<tr>
<td bgcolor="#13486D"><normalfont>Here you can invite your friends if they are not registred</normalfont></td>
</tr>
</table>
</td></tr></table>
<br>