If you want to be close to other guests, these hotels are in close proximity and have reserved rooms for this event:
Radisson Hotel Rockland
Best Western Rockland
Comfort Inn Rockland

These other options are not close to each other, but have some benefits of their own:
Wompatuck State Park (camping)
Sheraton Hotel Braintree (free shuttle to train, near mall & restaurants)
Nantasket Beach Inn (beach/special rate)