Work has begun to support OpenID Connect, which enables one to use any OAuth 2.0 identity solution. It is planned for Mattermost Cloud and Mattermost Self-Managed E20.
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I use Django's allauth module to authenticate my users. https://github.com/pennersr/django-allauth
This allows them to link various accounts - Facebook, Google, Github, StackExchange, Shopify and SoundCloud are the ones I use. Giving users this ability to connect from various points is helpful because they can connect how they want. From an admin point of view, it takes a little bit of work for initial set up, but once done has been pretty painless.
There is a community integration with GitHub here: https://github.com/softdevteam/mattermost-github-integration