https://slack.zendesk.com/hc/en-us/articles/203772216-Using-...
https://auth0.com/docs/identityproviders
Not sure about authorization and group membership, channel authorization etc -- I assume you'll need to manage slack authorization with slack, and github authorization on the github side.
But fair's fair.
[ed: And now you have three providers:
https://auth0.com/pricing ]