In the "My organization" menu, click "Add" on the top right corner, then on "Group".
Then go to the "My users" section to place the users of your choice in the group you have just created. The choice is made using the drop-down list in the "Group" column.
A notification appears at the top left of the interface to inform you that the change of group has been taken into account.
Tip: You can change users of group by selecting them and use the group action button.