Guys just do "THIS"
Find in the usercp_invitation template :
Code:
<if condition="$show['invitation_form']">
<form action="profile.php" method="post" name="vbform">
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="do" value="invitation" />
add below:
Code:
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
and update XML (Attachment)