add a style to the other text box to hide it:
<input id="Contact_Us_group_Other" name="Contact_Us_group_Other" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("index","Contact_Us_group_Other"):"")); ?>" class="formTextfield_XSmall" tabindex="4" style="visibility;hidden"/>
Click on the select list in design view, them open the behaviors panel, Window -> behaviors.
In the behaviors panel, click the plus button and select WA Validation Toolkit -> Client Validations -> Required / Not Blank / Selection made.
make sure to apply the validation to the select list, and make sure it is applied to the onChange event.
Click the plus button again, and select WA Validation Toolkit -> Show Hide Layer.
Set the trigger form element to the Select list. Set the target form element to the other text box and set the If trigger is valid option to Hide the target layer.
again, make sure this applied to the onchange event of the select list.