Well that isn't as easy as it sounds, Hermitcraft doesn't have an application system, it isn't a public server, but a private group of friends. Also due to Hermitcraft's popularity it is very rare for a new member to be added.
What do you need to join?
- An invitation by one of the Hermits
- To be very good friends with the members of Hermitcraft
- Have a semi popular YouTube channel that has consistent uploads and are passionate about Minecraft and creating content
- ALL the Hermitcraft SMP members have to say yes in a vote
There is a very small chance Hermitcraft will accept a random viewer/fan. Instead of trying to join Hermitcraft, it is better to create your own Hermitcraft or join someone elses Hermitcraft. Xisumavoid himself recommended this option instead of trying to join Hermitcraft on various streams, and in all his Q/As The "Hermitcraft like server" genre of Minecraft servers is a very popular trend in recent months and lots of server owners are trying to emulate the Hermitcraft Like feel so it shouldn't be hard to find a community where you can get the Hermitcraft experience.