Dominions' most thriving community is currently on discord. Discord is a website which functions similarly to a feature rich IRC client. There are many discords related to dominions, both general ones and specialized ones.
General servers feature channels dedicated to Dominions strategy discussions, active games, and non-Dominions related talk. These servers tend to be large, active, and act like hubs for other dominions activity.
The largest and most active general server is known simply as Dominions and is run by Althaea, who moderates just about everything. As the largest Dominions discord you can find more than a little bit of everything there, from noob games to mod strategy to book clubs!
The next largest is Clockwork Hounds hosted by Drithyl, creator of the Mr. Clockwork bot which has revolutionized the world of Dominions game hosting. This server is no more focused than Althaea's discord but has by far the most active blitzing community.
After this is Reddit Dominions which is most notable for having an outrageous concentration of high concept meme games, having given rise to the entire Aleph series which has inspired many spin offs and derivative meme games.
The last general server of note is the very official discord server, often referred to as the ruby server. This server mostly caters to something awful community and their strange and alien memes (see clowns.dm)
The specialized Discord Servers cover a broad variety of niche topics, ranging from the personal servers of major youtubers to servers for major mods to servers dedicated to specific games.
In these servers you can chit chat with your favourite youtubers (not that you can't do that on the other servers or in direct messages).
Hellenika is a greek themed mod pack chiefly by Aethernomad. The discord for it plays host to a surprisingly active noob modding community
Legendary by Executor is one of the most complex and opaque mods ever created adding thousands of events to spice up a regular game. This server pulls double duty for both it and Executor's other mod pack, Confluence, a combination of all of his mods.