Typically, Richard Branson’s Necker Island must be reserved in its entirety for one group. Later this month, however, the 74-acre private-island resort will host a seven-day Celebration Week package, in which all of the property’s 15 authentic Balinese-style rooms can be booked individually. Accommodations include the six one-bedroom Bali Houses, three of which offer sweeping cliffside views of the Caribbean Sea, and the nine rooms, each with an en suite bathroom, within the new Great House, which reopened las…