Setup a Slack auto-invite system (Want to join our Slack? Post in here)

Making this a dual purpose issue.

1. Who wants to setup a Slack invite system for us? Unfortunately you can't make a team Public. Two options were suggested in IRC (https://github.com/rauchg/slackin or https://stacktodo.com/tools/invite), but I'm open to anything that works.

2. Post in here if you want an invite to Slack. I'll need your email address, so if you don't want that public either make a note and I'll remove it after I add you, email me rappo@bountysource.com, or catch me in #Bountysource IRC Freenode.

Start by comparing the two suggested options, slackin and Stacktodo Invite, and determine how either could support the requested Slack invitation flow. The issue provides no repository entry point or test location, so first identify where such an external system would integrate. Done means a working invite system is selected and the Slack invitation process is usable.

Written by the indexing model from the issue text.