How to Add Customer Group field while register in magento

It's default magento Features.You need to Edit 2 file. one is config.xml which resides in app/code/core/Mage/customer/etc/config.xml and other is Register.phtml which resides app/design/frontend/your_interface/Your_theme/template/customer/form/Register.phtml

Open Config.xml then goto line number near about 82 there add <group_id><create>1</create><update>1</update></group_id> under
<fieldsets>
<customer_account>
The code will be something like this

<fieldsets>
<customer_account>
<group_id><create>1</create><update>1</update></group_id>
</customer_account>
</fieldsets>

Now go to register.phtml and write the following code under the section where you wanrt to put.

<li>
<label for="customer_groups" class="required"><em>*</em><?php echo $this->__('Customer Groups') ?></label>
<select name="customer_groups" id="customer_groups" title="<?php echo $this->__('Customer Groups') ?>" class="validate-group required-entry input-text">
<?php $cu_groups = Mage::helper('customer')->getGroups()->toOptionArray(); ?>
<?php foreach($cu_groups as $cuGroups){ ?>
<option value="<?php echo $cuGroups['value'] ?>"><?php echo $cuGroups['label'] ?></option>
<?php } ?>
</select>
</li>