Dashboard is user specific.
End users see the leads they have added/uploaded themselves. Or someone has assigned the leads them.
Eg. John uploaded 10 leads and assigned it to assistant Ashley.
Jack uploaded 15 leads and assigned it to Assistant Ashley.
John will see his own 10 leads in the Dashboard. Jack will see his own 15 leads in the Dashboard, even if the same Ashley assistant is working for both of them.
Assistant Manager is a role where the person can see all the leads that are being worked upon by a given assistant.
Assume Ryan is an Assistant manager for Assistant Ashley.
Ryan will see all leads Ashley is working on, irrespective of whether they are uploaded by John or Jack or Ryan himself.