Currently, it is rather hard for Private servers to maintain a good playerbase, and much of this can be attributed to the lacking of advertisement they have in both the game and the Launcher.
Now, improving this in game is a slightly harder task, but it is reasonably easy to fix in the Launcher. Right now, to find the different groups and servers you have to click a minor and non-descriptive button to the right of the Launcher. Not only is this very small, but it doesn’t even appear to be a button at first glace.
To improve this, I believe a Server tab is necessary at the top of the Launcher next to the Shop, etc. This would be a lot more obvious and would encourage more players to apply for Private servers.