Hi @Rampravesh Chauhan,
You have to add that user into that specific project role.
You don't. Projects contain users, users do not have projects assigned to them.
Go to the project, and, as an admin, add the user(s) to the relevant role(s)
Are you looking for someone to be 'in charge' of a project? If so then making them the Project Lead would be the closest.
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