Copy user permissions from database to named instance

A client recently created a new named instance on his SQL Server and migrated a database from another server to that new named instance.  When he did so, users were carried over from the old server/database to the new database when that database was restored, but the instance level logins were not copied over, and when he tries to create the user in the named instance, he gets a message indicating that the user already exists.

Does he need to remove each of the database users and add them at the instance level, and then assign them to the database?  or is there a way to script those users from original SQL Server database and create them in the named instance?