How do I restrict user permissions such that they can only access 1 project?

[Alifya Kagalwalla]

We have contractors who need to access 1 project in JIRA but should be restricted from viewing other projects.

[Jack Brickey]

Place users in a group under User Management > Groups and then use those groups in the permission scheme for the specific project. I expect everyone is showing up under a default group, e.g. Jira Users or something.