Hey there, for all newbies like me (and people who have a level too low to post here) that want to join rainbow void, here's a tip, because the square enix isn't the best dev to make a user friendly experience to join a free company.

- visit the rainbow void house at gridania's residential area (Plot 29, 5 Ward, The Lavender Beds)
- outside the house you'll see a placard, interact with it, there's an option that leads to a screen which has an application form.

That's how I did it.