Restrict contacts available to be assigned to an activity
The EntityRef field on the form can be given that restriction. Best way to do it is probably via a hook, e.g.

hook_civicrm_preProcess($formName, &amp;$form) {
  if (is_a($form, 'CRM_Activity_Form_Activity')) {
    $form-&gt;_fields['assignee_contact_id']['attributes']['api']['params']['group'] = 1;
  }
}


(where "hook" is the name of your module and "1" is the id of the group)

Note use of is_a rather than the $formName so we also catch the case activity form.