Log into Jira or Confluence and create an Application Link from Jira to Confluence (https://confluence.atlassian.com/adminjiraserver/using-applinks-to-link-to-other-applications-938846918.html).
Create login credentials for the Confluence server in the Jira User management settings, so that Confluence can establish a connection to the user directory of Jira.
Activate the checkbox External user management within the Security Configurations.
Add the Jira User Groups that should get access to the Confluence instance in the Global Permission settings in Confluence.
Add external gadgets like the Filter Results Gadget from your JIRA to your Confluence in the External Gadget settings. Therefor choose the Gadget Feeds option and type in your Jira URL.