I just wanted to share how you can put an automated Slack join-up behind a 'paywall'.
If you don't know about Slack, here is a chance to find out more anyway. (the simplest way to describe it is a private twitter for teams).
As Slack is a team focussed product, traditionally you have to manually invite new users.
However, you can automate the join up process and in effect make the 'team' public, so you can build a community by subscription.
Using this plugin (that I wrote, with a little bit of help from others) https://wordpress.org/plugins/fullworks-slack/ you can create a join-up form widget.
Now if you publicly display the join-up form you will have no real control over who joins your Slack team (community). That may be OK if that is what you want.
But if you want to protect the form, the plugin does have an option only to display to logged in users.
However if you want sign up forms and more controls that is where Membership 2 plugin https://premium.wpmudev.org/project/membership/ steps in.
By using Membership 2, you can create signup forms and put your community sign up behind a membership wall - a free-wall or a pay-wall.
I have implemented a demo here http://fullworks.net/slack-demo/ which includes a demo Slack team so you can try Slack for real. (Free of course)
Please let me know if there any technical hiccups, and let me know if you do set up something like this for real.