Two ways. The recommended way is to navigate to the toon via the URL: site.com/toon/[realm]/[toonname]. This will load a copy of the toon into the local database. Click 'claim toon' at the bottom of the page if you have permission, and a node will be created and assigned to your user profile.
For example, to view the toon Muertee on the Realm Sen'jin, you type the following in the address bar:
Also, from the toon page, you can naviagte to the guild that toon is in my clicking the guild name under the toon's name. You can also navigate directly there by going to the URL: site.com/guild/[realm]/[guild name] (real life example: http://synthesis.graviteedesigns.com/guild/Sen'jin/Synthesis).
[note: the following method has been removed] You can also go to Create Content -> WoW Toon and enter the toon name and realm. The authored by will be the owner of that toon and will display under their account page. So if you need to change the owner of a toon, you can edit it and change the authored by field.
If you are running a guild, you can go to /admin/content/wowguildroster and you will be able to assign members of your guild to specific user accounts.