How to figure out attendees time individually?

If you have a paid account, the attendee report can tell you when each participant arrived and left the meeting, as well as their overall duration. This article can show you more about Zoom Reports: https://support.zoom.us/hc/en-us/articles/201363213-Getting-started-with-Zoom-reporting 

Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.

Fundamentally the solution is simple - use a separate sign in sheet for each session and have people write down their name and the session they plan to attend. But if you want to make this process easier, you can use an Attendance System. You can either use a physical system or an online system.