Zumpul users are based on the Google Directory in your domain, so that if you want to add a new person to set a signature, you have to create it first in your G Suite account, when the users sync you'll see the new user(s) in Zumpul and thereafter ...
Nothing, the signatures that you applied are not affected if you unistalled the app or if you move from one version to another. (End users are able to change from their personal settings).
Make sure that all your staff has the proper signature applied, with Zumpul you can set a schedule where the signature will be applied automatically on a timeframe that you choose: weekly, monthy, etc
In Zumpul you have a powerful interface to modify a user's profile, any change that you apply in Zumpul is synced to G Suite and viceversa in order to keep the signatures with all the details updated. Working with users 401 - Update a users profile ...
You can use Zumpul to edit a person's profile, every change that you perform from Zumpul is synced to the Google Directory. This changes will reflect in the signature once you save and apply it.