Plugin "Add existing users" gives fatal error

"Plugin "add existing users" installs seemingly ok, but after entering email addresses for existing users, then clicking "submit"
I get the following error message:
Fatal error: Call to undefined function add_user_to_blog() in /hermes/bosoraweb119/b2313/ipg.barryclemsonnet/wp-content/plugins/add-existing-users/add-existing-users.php on line 322

Help, please!