I have installed XRay application in my Jira instance. I want to restrict the usage of this application to a particular user group. Please tell me the procedure to do this.
You can't limit access to an app by group in Jira.
If the app has been built to do it, then you can. I seem to remember XRay mostly inherits its access controls from the Jira project you are in, because the underlying tests (cases, executions etc) are mostly built on issues.