<div class="form-group field-signupcompanyform-phone">
<label class="control-label">Телефон</label>
<input type="text" name="SignupCompanyForm[phone][]" class="form-control">
<div class="help-block"></div>
<input type="text" name="SignupCompanyForm[phone][]" class="form-control phone-group">
<input type="text" name="SignupCompanyForm[phone][]" class="form-control phone-group">
<input type="text" name="SignupCompanyForm[phone][]" class="form-control phone-group">
</div>
делая
$model->load(Yii::$app->request->post())
заполняет все свойства кроме $phone