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March 6, 2023 edited
Hello Vikas,
Sometimes the user only had the access suspended, in case you are an org admin, you can restore his access by following the next steps: Administration page > directory > users > Total users (note that the "Total users" number will be different from the "Active users") > Restore Access (under the Actions column).
Now, if the user was really removed from the instance, you should be able to invite him again. Also, if you are a project admin and just trying to add existing Jira licensed users then you can do that via the People under your project.
If you are not an org/site admin, contact your site admin.