http://keybase5btp2qikoau6u326pvotn47gb55mo6fjxxyxdoywiinomr5id.onion/blog/introducing-keybase-teams
When you create a team, you begin a chain of signatures. Your first link declares that only you can append to the chain. You can then add other people. And technically, people themselves are chains, so you're signing their chains onto the team chain.