If for some reason you don't signup the first night, check with guest services after. For sailings with few at-sea days, the sessions fill up quickly, on a TA cruise, you may be able to join at the last minute. One note, you asked about setting up a class, Disney will provide this service if setup in advance and the minimum number of folks is usually 20 - 25. As another poster mentioned, if you are part of the meetup group, see if you can join there.