I believe that IF permits are available for the same day, that you can add permits onto your existing trip. But you have to watch the site and hope slots come available. To make sure, though, you can telephone the Wilderness Permits number 760-873-2483, and talk to the people there. They are the ones who know the full story.
Check often on May 1 as all the slots won in the lottery that were not paid for will come available (at random times through the day). If you are not successful, a few people will cancel their slots in the week or two before the date.
And finally, you are practically guaranteed that you can add people to your permit when you go in and pick up the actual permits. There are always cancellations and no-shows, and they become available immediately during the pickup time at the Visitor Center.
