Apps > Additional Google services > Settings for Early Access Apps > Core Data Access Permissions -> Allow users at your organization to access Google Workspace and Customer Data using Early Access apps.
This doesn't enable Bard per-se, but enables your Workspace for these new AI things, which should include Bard.
I can't access Bard on my domain account due to the existing country limitations (Germany), and I haven't tried over VPN (which I won't try).
And as another commenter predicted, searching the admin console for "Bard" turns up no settings.
Here is the URL for this and it specifically mentions Bard:
https://apps.google.com/supportwidget/articlehome?hl=en&arti...
EDIT: it's working now, took about 10 minutes.
In my case enabling these two things was nearly instantaneous, it told me it will take a couple of minutes (located in Germany).
A sibling comment to this one contains the link to the help.
Sorry for not providing the direct link to the setting. Even though my domain name was not in the URL, there was a long number in it which might be personalized (albeit with a very low probability of this being the case).
My entire family has been using custom secondary gmail accounts JUST TO USE GOOGLE FI.