You can delete groups from the local directory on a Yealink T19P by following the steps below.
With the phone displaying the Idle screen, press the Dir soft key
Select the desired group
Press the Option soft key and then select Delete from the prompts list.
To delete all groups in the local directory, you can do this by pressing the Option soft key and then select Del All.
Press OK to proceed with deleting the group or press cancel to go back without deleting the selected group.