I enjoy using webhooks and automation platforms, so here is how I’d do it on the quick!
- Set up webhooks, one for joining a group in Discourse, and one for leaving the group
- Catch the webhooks on an automation platform and format them for Nextcloud; I use huginn for this
- Create users/add to groups via Nextcloud API
This way you invite or remove users from the Discourse group to manage the corresponding Nextcloud group. There are some decisions to make regarding user accounts, passwords, etc. This is a quick way to get folks access to an external site from Discourse. 