Issues are imported per project, a project should be specified by its projectID. When issues are imported all their atributes are imported, including users (from Reporter and Assignee issue fields) and custom fields.
Notes about permissions, imported user accounts and custom fields
A user account you use to log in to source YouTrack server should have enough permissions to read issues and all their attributes including custom fields for the project to be imported. A user account you use to log into target YouTrack server should have enough permissions to create new project, new user accounts, and custom attributes, i.e. it should have administrator permissions. We recommend to use administrator accounts for both source and target servers.
When a user account is imported, it will be created with the new randomly generated password. That is, when a user logs into the target YouTrack server with his username from the source server, on the Login page he should type his username and then use the Send password restore E-mail link to restore the password.
Only those custom fields that are used in the target project will be imported from the source YouTrack server.
To import all issues in a project from one YouTrack server to another: