How to set a custom group of users

You can add a new group from the Magento admin area > Customers > Customer Groups > Add New Customer Group.

Once a customer registers, you can change the group he/she belongs to from the Magento admin area > Customers > Manage Customers. Click on the Edit link next to the customer and change the group from the Account Information > Customer Group. Click Save Customer.

Set the discount from Promotions > Catalog Price Rules > Add New Rule.

In the Customer Groups select the customers' groups for which the promotion is valid. Enter the other details, set the rule actions and conditions.  Finally, click Save Rule.

The above ten tips will hopefully help you resolve at least some of the Magento issues you have faced or are about to face.