The Global Participant Search feature is located via the Compliance Menu> Tools/Settings> Global Participant Search.
This feature allows you to search for a participant across all the plans under your account. You can search by last name or SSN/Employee Number, and the search covers participants listed both inside the census grid and below it. The results will appear in a drop-down, and you can select a name and click the 'Go to Plan' button to go straight to the plan that the participant is in.
When searching by SSN or Employee Number make sure to select the appropriate radio dial prior to selecting the search button: