You should start choosing a server, and then a kingdom from which you want to operate. The tribe might be important for the latter decision. And then start recruiting people to your guild.

Good luck,

Kant

You can't. There is no game for now.

However, as said above, you can create a discord server, or any equivalent and start recruiting. In this case you may also want to write a concept in the forum under the category Servers/Selene organization (or other server name, depending on where you lives irl).

Besides that, you can also use the recruitment channels in the official discord

Will a guild be the default for any organization? If I wanted to start a Knightly Order would I have to use the guild mechanic or is there another mechanic they'll implement?

There are three types of organizations, Guilds, Schools, and Associations. Guilds are for crafting mostly, schools are for teaching and researching, and associations is the catch all for everything else. Your Knightly order can by an Assoc, and run a school etc etc its a pretty fluid system overall.

I think you may be able to form a guild officially once they have the personas and organizations update to the website. But that's assuming their plans haven't changed in that regard, which may be a bold assumption.