The readme on https://github.com/strukturag/nextcloud-spreedme says this:
Limiting access to this app
By default, all users who can log in to your Nextcloud installation can also use this app (and Spreed WebRTC).
If you want to limit access to this app (and Spreed WebRTC) only to a selected user-group, open Nextcloud’s user configuration site in your browser: /index.php/settings/users.
Create a new group there (e.g. Spreed.ME). For sure you can also use an existing group (like admin).
Now go to Nextcloud’s app configuration page /index.php/settings/apps, find this app and check Enable only for specific groups. Then add all groups which should be able to use this app.
All users not in a group specified in the step above, will neither be able to use this app nor Spreed WebRTC.
###Access by non-Nextcloud users
If you want to use Spreed WebRTC with users who do not have an Nextcloud account, you can enable the “Temporary Password” feature on the Nextcloud admin settings page.
This allows them to use Spreed WebRTC with a “Temporary Password”, which admins can generate by clicking on the key icon in the room bar of Spreed WebRTC or at /index.php/apps/spreedme/admin/tp. Admins are either Nextcloud admins or Spreed.ME group admins. Create a group named Spreed.ME and add users as a group admin for that group to allow them to generate “Temporary Passwords”.
For a registered nextcloud user other than admin it appears the group needs to be named “Spreed.ME” exactly in order to see the key Icon and invite external users.
However…When you want to be able to invite external users, you cannot limit the app to a group.
External user will see a “message app not installed”.
The trick that seemed to work for me is to create the group “Spreed.ME” and allow all to be admins on that group indeed, but not limit the app to that group.
In my small setup where all can have access to the app, that is ok. Everyone that is admin in Spreed.ME now has the ability to invite external users, external users have access…
But I can imagine in larger setups, you would like the app restricted to users, but enable those that can access the app to invite external users.
I am facing this issue still though: https://github.com/strukturag/nextcloud-spreedme/issues/84