Right click on a student icon and select the Move to session option followed by the group of your choice.
How to put students into different groups 1

The student icon will change colour as shown below to indicate what group that student now belongs to.
How to put students into different groups 2

You can now use any of the whole class features described in this document to the students in a particular group only.
For example, clicking the blue head icon next to the Talk icon will mean that only the blue students will hear you and not all the class.
How to put students into different groups 3
Or perhaps you might want to conduct different activities with different groups?