I see your question is marked under Jeera service management. As such my first question is are you looking to add a customer or an agent? In either case when you add a user you add them to the application but you can also limit access to a single project. this is all managed via roles, groups in project permissions.
Welcome to Atlassian Community!
Yes, this is possible. What you have to do is change the permission scheme(s) that your current projects are using so only users belonging to a specific group or project role can browse the project, I would recommend using project roles, you can then control it based on the project instead of having to have multiple groups. You can then add the people that should only access to the one project to that role. You can find more information about how permissions works here, How do Jira permissions work?
For JSM June Challenge #2, share how your non-technical teams like HR, legal, marketing, finance, and beyond started using Jira Service Management! Tell us: Did they ask to start using it or...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events