Fortnite Duo Pros?

How does it happen that e.g. Ninja plays with Reverse 2K even though he only has 16K subscriptions? So not so well known. Are they then writing or how can such a small Youtuber duo become players of Ninja?

By knowing each other privately and getting along with each other

For example, Do Reverse and Ninja or Daequan and Ninja exchange their WhatsApp numbers? And what if they live far apart, then they can't say the numbers privately, do they do that through social media?

Be determined through social media or through other friends