When you click on User activity link on the left panel of your homepage you will see a list of user activities with their names and the project name to which they are assigned. Under activity column, you can view all the activities users are performing.

To find a particular activity of a user, you can use advanced search also.

Search can be conducted by different methods, for example, you can search based on dates or you can search user activity by selecting a project from the drop-down menu.