Rooms do open up
Rooms tend to open up about a week ahead of time in Yosemite Valley, which is the final opportunity to cancel without cost for people who have previously reserved space. It's useful to begin checking daily about 2 weeks out.
Getting 5 rooms likely would be a challenge, however.
Cancellations happen because people reserved early (given the popularity and limited number of rooms) but then have a change of plans.