Hi Rajesh, welcome to the Community!
User Management is done for entire Cloud instances at a time. You'd need the Site Admin permission to manage users in Jira/Confluence Cloud (users/groups are shared between the applications). Someone with Jira Administrator permission only would not be able to modify users for the site.
Hope this helps and let me know if you have more questions!
Cheers,
Daniel | Atlassian Support
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