In Time Spent Panel

... where you will be able to display logged spent time by different criteria.

Through Workers CustomFields

These 2 customfields will show who is working on your issue. They will too allow you to search for an Issue where a user is/has worked on.  

In Worklog Reports

Few Worklog Reports are provided allowing to aggregate Worklog based on different criteria.

• Worklog Reports Concepts — All provided Worklog Reports share some common concepts :... Aggregation, Hierarchie and Flat views 
  • Third Part Aggregation Criteria
• Available Worklog Reports — It allows you to append, as allows JIRA since 4.2, a Log Work Field in Transition Screen. Even if it is a native feature now, you will need to continue with it if you want to benefit from Worklog Type.
  • Workload Report (Worklog Type)
  • Workload Report - Issue Type
    • Workload Issue Type in details — It allows you to append, as allows JIRA since 4.2, a Log Work Field in Transition Screen. Even if it is a native feature now, you will need to continue with it if you want to benefit from Worklog Type.
  • Workload Report - Project