The Customers tab lists your most active customers and tracks the volume of customers you have been able to help (that you have replied to) in the date range - see each linked metric below for details:
Report Dashboard - Customers Tab
The list of customers is populated from the saved contacts in your contact manager, excluding private contacts. A customer must be a saved contact in order to show up in this list.
If your teammates' private inboxes have been made available to analytics, then the private messages they send to saved team contacts will also reflect in the team analytics report's Customers tab. However, their own private saved contacts will not show up in the Customers tab.