In order to allow all of your guests access to every session, you must set the appropriate filter.
If the filter section is left blank, no guests will be allowed to attend that session. You must put every registration type or custom question answer to allow access for every guest.
Go into Manage Sessions section on the left hand side of the screen.
Next, scroll down to Access Control and switch the toggle on.
Finally, switch on the Filter Options.
Once this is complete you will be able to give guest access to register for the sessions.