Select Participants from the secondary menu.
Select Groups from the tertiary menu.
Select the group you want to add participants to.
Click the Add/remove users button.
Select Participants from the secondary menu.
Select Groups from the tertiary menu.
Select the group you want to add participants to.
Click the Add/remove users button.
You will be taken back to the Groups tab.
Repeat this step as necessary to add students for each Group.