I know that it is possible to run multiple instances of discourse on the same server using a multisite setup. I’m wondering if it would be possible for multiple instances of discourse to make use of the same user database? So that a user could sign-up on any one of several different forums and use those same credentials to log in to all of the others. If a user’s whole profile (trust level, profile detail, pictures, etc) could be synced across each instance, this would be ideal for our use-case.
AI-generated summary
The discussion revolves around using the same user database and login credentials across multiple Discourse instances. auryn_macmillan inquires about the possibility of syncing user profiles and credentials across instances. fefrei suggests setting up single sign-on (SSO) between instances. However, auryn_macmillan is looking for a solution that uses Discourse’s authentication instead of a third-party service.
codinghorror proposes designating one instance as the master authentication site. auryn_macmillan confirms that they do not have a parent website, making this setup necessary. sam confirms that Discourse can act as an SSO provider for another instance, and provides a link to the relevant documentation (https://meta.discourse.org/t/using-discourse-as-a-sso-provider/32974).
auryn_macmillan successfully enables SSO on their test forum, using their main forum as the SSO provider, and finds the user experience to be friendly, even without explicitly stating the hierarchy.
You can set up single sign-on between both instances.
This would require us to use some 3rd party authentication correct? We’re quite happy with Discourse authentication, would it be possible to use Discourse’s Auth from one instance as SSO for the other instances?
You would be using your own auth, from your master website.
(Do you not have a parent or master website people already log in to?)
No, the only portion of our website that requires auth is Discourse.
Aha in that case one of them will have to be the master auth, I suppose. Any thoughts on this @sam?
Sure, Discourse can act as an SSO provider for another Discourse instance.
Awesome, is there any documentation that for setting something like this up?
Follow the breadcrumbs 
I still think this is gonna be weird since one of the Discourse sites – and how do you tell the user which one is the “right” one – will be the master auth site. There still needs to be an understandable hierarchy here of some kind.
I’ve just enabled it SSO on our test forum, using our main forum as the SSO provider. Even without explicitly stating the hierarchy (which may be necessary if we start implementing multiple instances) signing in seems pretty user friendly.