CRM Core provides the ability to link a contact record to a user account. Linked records are stored in the relations module, and will allow developers to load a linked contact record given a valid user account.
Enable CRM Core User Sync to enable user synchronization management. You can access user synchronization options through the configuration section of the Drupal administrative interface, under CRM Core, by selecting the User Synchronization.
User Synchronization is for associating user accounts with contact records. It does not care about what contact information is already in your system. It creates new contact records and links them to existing user accounts, and allows you to explicitly define those relationships through a UI. It also allows you to specify the rules for creating contact records when new user accounts are created.
This is not an automated contact matching tool for CRM Core, it will not attempt to link users and contacts through an email address. That is another challenge not addressed by this module.