Who? There are two ways of gaining the opportunity of using MM.
1) Buying ANY ticket of 3 or more days. This will entitle the ticket holder to one MM admission. There are some exceptions such as Gay Day, military, and convention tickets, but most big name ticket sellers have MM eligible tickets (Disney, AAA, Costco, So. Cal City Pass, most online sources such as ARES Travel)
2) Staying at one of the 3 onsite hotels (Grand Californian (including the new VGC DVC places),
Disneyland Hotel, or Paradise Pier Hotel). This will entitle the hotel guests MM admission every day that it is offered during their stay, including arrival (yes, possibly before your room is ready) and departure days, regardless of where you buy your tickets from. For example, if a family was staying at one of the onsite hotels for a long weekend (visiting the parks Saturday, Sunday, and Monday). They would be allowed to use MM on both Saturday and Sunday; however, a guest staying offsite would have to pick one or the other.