Event Management -> Guest Groups

Start by clicking on Event Management on the left hand side menu and click on Guest Groups

Click on the Guest Group name to show the Group details: 

Click on Guests located on the Guest Group page to display the Guests who have already been added and add new Guests:

After clicking on Add Guest, a new window will show you a list of Guests that can be added to the Group. From here, you can select Guests to be added and click Add Selected Guests

All Guests will then be displayed in the Guest Group details page after being added and you have successfully added a Guest to a Guest Group: