There are a couple of different ways to assign guests to each event/guest list group. 

From the edit/view guest option there will be an assignment/RSVP tab where you can assign each guest.

You can also choose which guests to assign to different guest list groups in the main page. You can find out more about how Guest List Groups work here - https://gettingmarried.co.uk/guest-list-groups-user-guide/