To complete the Forza 6 Community Welcome Achievement all you have to do is click on career and select the forza hub and you will then complete the Forza 6 achievement community welcome achievement. But this could be one of the most useful achievements as you get free money and bonuses for using the forza hub you can get millions a month depending on your tier and level
So be sure to check the Forza 6 Hub on the 18th Of September