Well, re: authentication -- I didn't think to check slack's support for oauth etc... apparently there's 0auth.com:

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 ]