Button to join group on post

[ondrej]

The closest thing I can think of is this plugin

Discourse Policy

Summary Discourse Policy gives the ability to confirm your users have seen or done something, with optional reminders. Install Guide This plugin is bundled with Discourse core. There is no need to install the plugin separately. Features Discourse policy shares much of its DNA with polls, it has the goal of ensuring members of a group accept certain policies by clicking a button. It has some specific features that set it apart from polls: It must …

which allows you to:

@anon82911141

agree or something