Sharing depends on the kind of restaurant (you can't share at buffets, etc. except with an infant), although there's little incentive to do it if you have the DDP and have already paid for a separate meal for each person.
One of the best ways to maximize value is to avoid 2-credit restaurants (unless your bill is really going to be more than twice that of a 1-credit restaurant), and to eat at only the most expensive places (generally dinners at 1-credit character buffets or other all-you-care-to-enjoy 1-credit meals, and meals that include a parade/show viewing package, will be the best use of your DDP entitlements). Use your credits for lunch or dinner instead of breakfast, order only the most expensive items on every menu (note that pricey milkshakes and smoothies are often DDP-eligible "beverages"), use your refillable mugs to the maximum possible extent, and leave no credits unused. As far as I know, there's no tie-in to the MDE app (at least, there wasn't when we last used the DDP in 2016), but every time you purchase something on it, your printed receipt will show the number of credits you have remaining. You can make certain substitutions between drinks/snacks/desserts (check the plan brochure for the details - typically you can sub any item marked as a "snack" for any other item marked as a "snack," whether that item is a drink, dessert, or side, although you may be surprised at CS places how few options there are, and at TS places you can sub dessert for a side or soup), but you cannot sub for appetizers. Mobile Ordering is also now available for those on the dining plan.
Simply Google, "Best Disney Dining Plan Values in 2018" if you want to find out which restaurants are the most expensive/best value. There's also a handy tool at distripplanner.com that you can use to see average prices for meals at the restaurants you're thinking about, and compare the DDP cost for your dining itinerary with the out-of-pocket cost.