@gregcohn we would love to limit the permissions but we need Slack to offer narrower scopes before.
In the meantime, our T&C's and Privacy Policy only allow us to acces the data we need to provide you with the service.
My team and I have spun up a number of these shared channels over the past couple of months. After using in the wild, I truly believe this is a key piece of the Slack experience that was missing.
I wrote a few more thoughts about my views and how it works (for us), here: http://goo.gl/XnWpEu -- Hopefully Ernesto and Blanca from Slackline will jump into the comments later today!
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