Well, it is union members as in registered members of the forums/website with the title "Anno Union".
There are playtests to which you could apply and if chosen, then travel to Mainz, Germany to test the current game and provide feedback.
I guess what makes the Anno games so special is that they can appeal to a broad range of players. There are always some sort of story scenarios which you can play, but most time is usually spend in the "Endless" game modes, or challenge scenarios.
In the "Endless" mode you have all options in your hand and you can configure the game to your liking. The core gameplay loop is rather simple. Fulfill the needs of your citizens at the current level, let them upgrade to the next level, repeat. I think this simple idea evolves into a very rich experience. With increasing rank of your population demand for new production chains always give you new incentives to keep settling on new islands. The production chains require transporting of goods over larger distances.
I like to play it slow and relaxed. I am trying to build large cities, not necessarily the most beautiful ones. Huge islands and a lot of resources, yes please. Mild pirates and one AI player for a hint of competition and need for defenses. Garnish with possible fires and sickness. Voila, I am happy
The setting of the older games (1602, 1503, 1702, 1404) scratch the European colonization itch, I guess
Go to a barren island and from nothing create beautiful and rich cities. Then kill all your competitors if you like
I only played a little of 2070 and it wasn't as much of a hit with me, but I can't really put a finger on a reason. If you want to start I would suggest Anno 1404, its often on sale at GOG for 2.5-5€ I think.
I think that the Anno games are very popular in Europe and especially in the german speaking countries is that the series was started by austrian and german companies. It also feels to me that strategy games in general are very popular around here, although I can't give you a good explanation for that.