og_activity depends on Rules

Basically the last funcion at og_activity.module calls rules but the module is not rules dependent

/**
 * Submit function to load the become og admin rule.
 */
function og_activity_create_admin_confirm($form, $form_state) {
  rules_invoke_event('og_become_co_admin', $form_state['values']['node'], $form_state['values']['account']);
}