There are many doodads already out there that allow the general public to invite themselves to your Slack instance. After surveying the range of options, I decided to write one of my own since many of the existing ones lacked input validation or did other silly things.

ANYONE with the URL can create invitations. Think and use accordingly.

Released to the public domain with no warranty whatsoever. All rights reversed, all wrongs forgiven.

• A webserver with PHP5 and PHP_CURL support.
• An existing instance on Slack.com
• The auth token from an ADMIN (not owner) on that Slack.com instance.

• Copy call-me-index.php onto your server.
• Edit the site-specific variables at the top, or (if you prefer) set them in a separate file ('vars.php' in the example)
• Optionally, add a background graphic (bg.jpg is defined in the CSS already).