Pleased to say there is a way to add a group of People who attended a historic session to a future one 

Go to Activities Tab

Manage Session Registers

Find the session you are wanting to move people into 

Click Search by Session 

Select Historic or Future (datewise) and then the Venue and Session that have your people in

Click Search - this will show you the people who had attended the Historic session - you can either select them individually or click All to bring all of them into the new session 

Click Add To Register