If you have more than about 35 guests, then the easiest option is to get a motor coach. If you only need it to go from the WP to the BoardWalk, you could save a few bucks by doing a one-way. But if you want it to pick everyone up for the ceremony too, they'll make you get a chartered motor coach at about $99/hour for a minimum of 5 hours. Then you'd just add hours if you want it to stick around to take people back to their resorts after the event (or during the event).
If you have fewer than 35 guests, it is much cheaper to get a handful of one-way vans at $40 each—especially if you only need them to go from the WP to the BW. It's also much more efficient for transporting guests to the ceremony and home from the reception because you can have one van go to X resort and another go to Y resort, instead of making your guests sit on one big bus for 2 hours while it drives to all the resorts getting people. BUT, if you need too many one-way trips, the vans start to add up to be not much less than a charter bus. Plus, some planners insist that you also hire a $200+ "transportation coordinator" if you get too many vans and too many trips. Disney prefers it if you just get the motor coach, but it's way too much money and vehicle in some situations.