You can add students to your account at anytime by purchasing subscriptions for your students. You can purchase student accounts with a credit card or PayPal by going to your Account Page and clicking the 'Extend your subscription/Buy Student Code' button and following the prompts. If you'd like to purchase student accounts with a purchase order check out this article for information on buying student accounts with a purchase order.
Once you purchase accounts for your students, you'll receive a Student Code to give out to your students to redeem and activate a subscription for themselves. Check out this article for more information on Student Codes.