When creating groups in Smart viewer they are named Group, Group(1), etc. by default.
To change these group names - click the gear icon at the top left and the Setup screen will open.
Click on the group to be renamed and press the F2 key on your keyboard
The selected group will turn blue and you can delete the group name and enter a new one - press enter to save
Close the Setup screen and the new group name is now shown in the drop-down menu.
This also works for device names.