You can't add another person to your group because there aren't any slots available within the quota limits. However, on May 1, any slots won in the lottery that have not been paid for will be released back to the online system. They release them throughout the day at random times, so you should check very often for the date you are trying to add.
If your entry date has an available slot, you should grab it as quickly as you can (as there might be others checking at the same time). Make sure you are logged in to recreation.gov already before you start looking for slots.
What date are you looking for, and is it day hike or overnight? I can set up a link for you to bookmark.