As part of our insourcing strategy, we are building our own development department. After developing applications with Visual Studio 2017, users are supposed to test the result. Do we need to purchase a license for these users?

A Visual Studio 2017 license (except monthly cloud subscriptions) contains the right to give up to 200 users access to an online demonstration of your development results. Moreover, you do not have to purchase a license for end users who are to test and review the software. In addition, you do not have to have a license for end users who use the software for the purpose of creating a feedback.

Source: Licensing Whitepaper_Microsoft_Visual Studio 2017_March 2017