How to add a unique domain(eg.www.ppp.com) to the network in MULTISITE(main domain www.aaa.com)

I am currently working on supergemsenterprise.net.

I have installed and enabled MULTISITE network.

I want to add a totally unique domain call supergems88.net to the multisite.

The reason to that is I want to share user logins. (means if user login any of the site, it automatically log them in the other site)

But how do I add another domain to the network?