If you want to add customer in different group during registration than follow this instruction.

You need to change in two files.
1) app\design\frontend\[your theme]\default\template\customer\form\register.phtml
add following code in the page

<div class="input-box">
   <label for="group_id"><?php echo $this->__('Group') ?><span class="required">*</span></label>
   <select name="group_id" id="group_id" title="<?php echo $this->__('Group') ?>" class="validate-group required-entry input-text" />
   <?php $groups = Mage::helper('customer')->getGroups()->toOptionArray(); ?>
   <?php foreach($groups as $group){ ?>
   <option value="<?php print $group['value'] ?>"><?php print $group['label'] ?></option>
   <?php } ?>
   </select>
</div>

2) app\code\core\Mage\Customer\controllers\AccountController.php

now replace $customer->getGroupId(); with

if($this->getRequest()->getPost('group_id'))
{
        $customer->setGroupId($this->getRequest()->getPost('group_id'));
}
else
{
        $customer->getGroupId();
}

This is good practise to override AccountController.php in your local directory