For creating a paid membership, I installed the plugin Memberful on my wp-site, that offers Discourse integration. The payment works in test mode, but the users are not added to Discourse.
Could these settings be the problem? I am not sure whether they are overridden by Memberful or I set them prior to using Memberful:
SSO is enabled, but there is still turned on: 1. Invite only 2. login required
and 3. ‘allow new registrations’ is disabled.