Start your class and click the 'Rearrange pupils' button.
This will open up the customisable optimise seating sidebar. You can now adjust the sliders to how you would like the pupils to be sat next to each other. Optimising for interactions will sit pupils who are good influences on each other together.
When the sliders are set how you want them click 'Optimise seating chart' to put the pupils onto the desks.