If a user is working on one project and the project is completed. So If i want to add/assign to another project how can I do it.

[Nic Brough -Adaptavist-]

There isn't really any question here, it's almost answered itself

Project completion is not a function in Jira, it doesn't matter, so:

• Add them to the project(s) you want them to be in
• Remove them from the project(s) they should not be in

[Nic Brough -Adaptavist-]

Not quite. The assignee is not "invalid" at all. If you remove the user and they have assigned issues in the project, it will NOT cause any technical problems. All that happens is that if you try to change the assignee, you will not be offered that user any more. It may cause a minor process problem - for unresolved issues, you probably want to reassign them to other users who will continue to work on the project. So it's worth looking at the list of *open* issues they are assigned to when you remove them from the project.

[Charles Wells]

Jose has included a few specific situations there. In general, as @Nic Brough [Adaptavist] said, you can add users to the desired projects via either Project Roles (preferable) or groups. It really depends how you've constructed your permission schemes.