I have many clients uploading to the same directory.

I have set chmod to 733 and chown to another user on the upload folder so that the different clients do not see each others files,
but that also causes the client to not see their own files after they are uploaded or not being able to resume broken uploads.

I would like the different user log in with their own username and password but all upload to the same directory and not see each others files, but just their own. Is this possible? Or does anyone have a tip how to achieve something similar?

The traditional way to manage this is to create a separate
directory for each account. If all of these directories are
in a single parent directory, it will allow a manager to see
all files in directories named for the accounts.

I'm wondering why you want to have all the clients' files in
a single directory, when a structure like this will meet all
of your stated requirements, except that they all be in one
directory.