Select the guest in the 
Groups pane and drag them to the appropriate group.
Alternatively you can use Edit>Change Group or set the 
Guests pane Mode to Edit and edit the Group column.
Note that changing does not change a guest's proximity for the automated seating. You can change the proximity of a couple to 
Next to by selecting both guests and then selecting Edit>Proximity>Next to.
You can delete any empty groups from the Groups pane once you have finished.
See also:
•Insert a new guest into an existing group