On the screen that asks how you want to receive your refund, after you select Direct Deposit or "Mail me a check," scroll down to the very bottom of the screen and check the box that says you want to purchase I bonds. Then continue through the screens and you will get to the screen for purchasing I bonds. The exact sequence of screens depends on which refund method you chose.
And yes, the $5,000 limit is per tax return, not per person.