Your main group doesn't suit you anymore? To change it, go to the "My organization" module:
Step 1: Go to the "My Organization" module
Step 2: Click on the group you want to set as main
Step 3: Click on "Promote" at the bottom right of your screen
Step 4: All you have to do is validate
A star now appears to the right of the new main group.
The users without entities that you will add later will be in this group so that they too can have a signature template.