Enterprise Edition currently supports SSO with ADFS, Okta, OneLogin, Google, Office365 and generic SAML. Generic OAuth2 is under consideration for Enterprise Edition as well.
Note: For a single-sign-on experience with Mattermost Team Edition, you can connect to other applications using Mattermost as an OAuth2 provider: https://docs.mattermost.com/developer/oauth-2-0-applications.html
This is now available in Beta in E10 and E20: https://docs.mattermost.com/deployment/guest-accounts.html.
An error occurred while saving the commentDaniel Stone commented
This can also be useful for customers/clients/externals, allowing them in so you can communicate with them and more easily share files or ideas, but without letting them roam into seeing things they shouldn't.