First things first. I don't think a useful answer can be suggested here until you work out the basic structural issues we are discussing here: Restrictions in total participants
For one thing, there will be just 1 field for registering for a session, but more than one record for a given participant to register for sessions.
BTW, you haven't mentioned "Classes" in your other thread. Please describe these in a post to the other thread as you will likely need yet another table for your classes separate from your Sessions table.