yes you can share your dessert. While they won't let you order a full extra meal & use a credit to buy a meal for someone not on the plan, you can order the meal you're entitled to and then share it with someone else.
We ate at Tutto Italia in January with friends...we were on the dining plan and they were paying cash for their meal. We ordered 2 apps (hubby & I were on deluxe dining) The server delivered the apps with 4 plates, obviously expecting us to share. The waiter did the same thing when we ordered our 2 desserts...brought out the 2 desserts with 2 extra plates and 4 dessert spoons. We did not ask for extra plates in either instance, and the waiter knew which 2 people were on the dining plan and which 2 were paying cash so he'd know how to split the checks. It was just obvious they fully expected sharing at the table.