It should be possible to use Zapier to add users to Discourse groups now, but it could be fairly complex to set it up. There are some details here that could be helpful: How to make requests to the Discourse API with Zapier.
My hope is that an action can be added to the official Zapier Discourse integration for adding and removing users from Discourse groups. If that is done, setting Discourse group memberships based on actions that occur on an external service will be a straightforward process. I’m looking into what we need to do to get that to work. I’ll move this topic to our #feature category and update it as I get more information.