I confirmed with a Disney wedding consultant today that the bus holds 47 passengers. It costs $88 per hour with a five hour minimum. That's why it's listed at $440 whether you use it for one hour or five hours.
If the bride and groom wants to enter Epcot or Disney-MGM Studios, they must take the back entrance. If the guests wish to enter the theme park, you can have them group together at the park entrance and Disney will have someone there (arranged ahead of time through your coordinator) to escort them to their destination. I confirmed that with the consultant today too. That means you may only have to bus everyone around, depending on convenience and cost.