I would like my clients to access a portal using some kind of authentification (their email address?) so basically access to their own secure client/customer portal and let them securely upload documents in a folder related to them. so my team on other side can check and accept or reject documents.

I am running a Immigration firm and has lots of documents to use in my cases to submit Immigration department, so need good platform where my team and client has better undersanding about whats going on 

I am trying to search but getting confused, I am also searching for third party integration for the same but not geeting any solid information.

all your feedback is appreciated