To group your leads by date of creation and then by salesperson, go to the menu Sales > Leads and remove any preselected filters. Click the arrow at the right side of the filter box. In the Group by list, first click Creation, then click Salesperson. Close the filter window, e.g. by clicking the filter arrow.

Notice that no lead is visible, but only one or more months followed by a number indicating the total number of leads for that month. Each month has an arrow in front; click the arrow to display all the salespersons for that month. Click the arrow in front of a salesperson to have a look at the corresponding leads for that month / salesperson.