Administrators and users who have been assigned the mass update permission have the ability to attach or remove a single user from either their entire lead list or a smaller filtered portion of their list. This filtered portion is referred to as a view.
To attach a user to an entire set of leads you must first be looking at your leads list (seen by clicking the Leads tab at the top right of your screen). Next, you will need to look at the view you wish to be seeing. For example lets say you want to assign your top salesperson to all your leads located within the view Hot Prospects. Choose the view "Hot Prospects" from the drop down menu located next to the heading Current View.
Once you are looking at the correct view access the dropdown menu located directly above the type column on your lead list. Choose the option "Assign/Remove user." From the Assign/Remove user screen choose which account user you would like to attach/remove from the set of leads by accessing the pulldown menu containing the current users on your account. Once you have chosen the appropriate user click the run button to perform the update.
If you would like to assign/remove a user to 20 or fewer leads on the leads list screen you are presently viewing you may also click the check boxes located on the far right side of your list. Once you have checked the appropriate leads follow the same procedure detailed in the previous paragraph.