The users who are just running reports need runtime licenses, which are free at this point in time.

Users who create or modify data items, joins and other system tables (including adding new users) need an Administrator license to do so. So purchasing Designing Administrators for them is the best solution.

In fact, most of our developer customers find that each of *their customer sites* benefit from having at least one administrator license at that site. This allows them (the developer) or at least one end user who acts as administrator to add or modify entries in the system tables directly. You could set it up so only you can maintain the tables, or allow someone else to do it in your place.