Again, the bottom of the “Current clients” section you will find a drop down menu with all of your sub-user's names. Select the user you want to allocate the lead to and click on “Allocate”.
You can also "Bulk Allocate" a selection of client to a Sub-User or transfer (bulk allocate) a selection of clients from one user to another. You can do this by checking the tick boxes of the clients you wish to Bulk allocate, selecting the sub-user you wish to allocate to and then clicking on the allocate button.