Option to hide drop down boxes for others

I want to customize my clients/external users/users view for them in the main window. Mainly, I want to hide the drop down box of users the one that lists "-all users-, -just mine-, -not assigned-, ."

I'm putting projects up for bid with multiple consultants, and I do not want the clients/external users/users, to know who else is bidding on the project.

Same goes for the "Groups" dropdown box. I would like an option to hide it for my users/external users/clients.
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