Hi

Have a look at: System Definition - Installation Exits

You should edit 'Logout' or if you have MultiSSO enabled 'MultiSSORedirect'. 

You need to edit this Logout entry for that. Like if(user==A){

var gotoURL = "url of specific portal";

response.sendRedirect("logout_redirect.do?sysparm_url=" + gotoURL);

return true;}

Something like this .

Regards,

Omkar Mone.

www.dxsherpa.com