We are working with external partners on a single jira project but we are having trouble restricting access to a single project. The existing documentation is for old projects and fail for next gen.
Here are the steps I tried.
1. Add users to jira
2. Remove jira-software-users (since otherwise they got access to all our projects) group for these users.
3. Go to project settings
4. Add these users by "add users/group" flow.
They are not able to see this project or any issues inside.