For the board, look at the sharing options and make sure they are only granted to the project, or a group with those six users.
For the project, it's a bit more complex. Look at the permission scheme for the project, and look at who can "browse project" - you need to make sure it is only your six users who can do this.
If you have a good scheme, it will probably say "browse project: role (users)". This is good because it means you can go to the "people" section in the project and remove any group or user you don't want to see this stuff (e.g. the "jira-software-users" group, which is the same as "anyone who can log in"). If it says something else, you're going to have to think a bit more about it.
Everything below is tested on Ubuntu 17.10. I prefer to use Jira in a docker container because: 1. I can install Jira with a couple of commands. 2. I can start and stop Jira just by starting and s...
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