It should be done automatically. I hope you don't have WP user role manager plugins and you've not changed something in WP user role settings. Also user registration should be done on checkout page. Only in this case wpForo detects and set him/her "Customer" usergroup.

Can you explain how to add a new field in the checkout page?
I have seen the reference article to add custom fields https://www.cloudways.com/blog/custom-field-woocommerce-checkout-page/ but I am having an error in this line of code
if (!$_POST['customised_field_name']) wc_add_notice(__('Please enter value.') , 'error');