If you use TweetDeck on a desktop, you can set up a "Team" where an employee has their own password, and has the ability to post to the business account, but does not have access to the password that can lock everyone else out.
But as far as I can tell, there's no way to do this on mobile. As they rolled out "teams" over two years ago, I assume they have no plans to fix this oversight.
I guess I could host my own access-controlled web page that allowed my employees to post text and images to a form, and then have it twit on the back end... but that would totally not work at all for video. (All of the social media apps resize the video locally first and then upload it in resumable chunks, and with good reason. You can't accomplish this sort of thing without a native app.)