You can get username and password of user's and use following code to directly authenticate in ELGG and use it's functionality.
//now authenticate user if not logged in
if(true===elgg_authenticate($SESSION['username'], $token_password)) {
$user = get_user_by_username($SESSION['username']);
try {
login($user, $persistent = false);
system_message(elgg_echo('loginok'));
//if admin then redirect to administration
if(elgg_is_admin_logged_in()) {
forward('admin');
} else {
forward('dashboard');
}
}catch (LoginException $e) {
register_error($e->getMessage());
//forward(REFERER);
}
}