If you want to get a fairly accurate estimate of people who will be attending sessions or workshops, then use the participant registration module of ConfTool to let participants register for these events.
Please skip to the next answer if you are only using the module for submission and reviewing of papers.
You find a manual on how to create the participant registration form here:
Configuration of the Participant Registration ModuleFor the sessions / workshops, first create new event groups here, representing time slots:
Overview => Settings => Manage Event/Item Groups Usually, only one session / workshop can be chosen within each time slot.
Hence, for the option "Selection Mode" choose the setting "Exclusive: Use radio buttons so that only one event of this group can be selected" (see image 1).
Then create individual events for each session / workshop here:
Overview => Settings => Manage Events and ItemsFor the option "Price Category" make sure to choose the setting "No costs apply" (see image 2).
Please be aware that radio buttons cannot be unselected, therefore create an additional event for people who do not wish to attend a session / workshop (see image 3).