After improving other aspects of the interface for the online lobby, I realized the invite buttons were not particularly intuitive. So, instead of different-colored plus signs, they now have proper text in them.
The user interface for the online lobby has been improved for enhanced appearance as well as better functionality (particularly, I have finally implemented the player list as a proper scroll box).
I have made some improvements to enhance performance, as well as to increase functionality of the user interface. And in case you missed it in the last update: this game has multiplayer support.