This plugin has been closed

As the Slack API has changed over the years, this plugin has become obsolete and lost its functionality.

With this in mind, and a lack of time, and dedication to this particular plugin, it has been closed.

The source code is still available via SVN on the WordPress plugin directory at https://plugins.trac.wordpress.org/browser/meeting-attendance-for-slack/ if anyone wishes to fork it, or work with it on their own.