In order for your users to retrieve their signature when writing a new email, they must be assigned to a signature template.
If a user is without a template in Signitic, at the next update, his signature will not be deleted but it will not be updated either so the user will not consume a license.
To assign a signature template to your users, go to the edition mode of your template and then to the Assign tab:
Here you will be able to choose for which users of which group and entity the template will be applied by ticking the corresponding checkboxes.
Don't forget to save and that's it!