Hi, Nico. Thanks for your note! Recruiting new vs. existing users for your research depends on your goals and what questions you’re trying to answer. The people you choose to recruit should represent the people you’re designing for. For example, if you want to test a design for a new OOBE (out of box experience) for your product, then you probably want to recruit people who are unfamiliar with your product. But if you want to figure out the best way to describe your product’s value proposition, you should probably talk to existing users about how and why they use your product.
This article has more details about recruiting research participants: https://library.gv.com/the-gv-research-sprint-start-recruiting-participants-day-1-8fa0bc1c8e71