Here’s what consumers have asked with answers from Grubhub staff and previous consumers.
Assuming you actually get your food (most people don't seem to actually get their orders), you won't have to tip after placing a tip on your order.
You actually have to wait a few days for the link that they send you by email. The email's subject will be, "How Was Your Order From_______?" I guess they don't want unverified customers leaving fake reviews, which is OK, but they sometimes do not send me the review email. You could also click the link to write a review for the previously ordered restaurant at the top of the home page after you log in, but watch out because this goes away after you order from another place or after a few days. They need to work on a better review system.