How many users are we talking about? You can see users who have only Trust Level 1 from the admin report /admin/users/list/newuser. This does not include users with TL1 or higher. From here you can manually up the user’s trust levels. You are correct that all users on the site technically have TL0, even if they also have a greater trust level. If a lot, a rails command is your best bet.
You should also make sure to change the site setting default trust level if you want all users to have TL1.