The standard functionality within Salesforce Health Cloud's Visit Management allows for multiple internal users to be associated with a single visit record. By using the "Add Visitors" action, the field sales rep can add the Medical Science Liaison (MSL) to the existing Visit record. This correctly represents the MSL as a participant in the same event and ensures their involvement is officially tracked against that visit, fulfilling the system tracking requirement without creating redundant data.

A. Create a new Visit record for the MSL: This is incorrect as it would create a duplicate, separate visit, misrepresenting that the MSL was a participant in the rep's original visit.

B. Send the meeting link to the MSL by email: This action occurs outside of Salesforce and fails to meet the requirement of tracking the MSL's participation within the system.

C. Add Visitors (Users) to a Visit: This is the correct answer.

1. Salesforce Health Cloud Developer Guide. "Visit Object". The documentation for the Visit object describes its purpose for scheduling and tracking interactions. The related lists and available actions, such as adding attendees (internal users as Visitors or external contacts), are standard features for managing participants.

2. Salesforce Help. "Manage Visits in Health Cloud". This documentation outlines the process of creating and managing visits, including adding attendees. The "Attendees" related list on a Visit record is used to track all participants, which includes internal users (Visitors) and external individuals.