Hey there, @edtowers , welcome to our Community!
Keep in mind that the Professional account is intended for personal usage, and not for sharing. With that being said, a Dropbox plan that uses 3 minimum users is our Business one. Which one do you use?
Can you check your plan page, and let me know more?
You also mentioned that the files can't be accessed by the 2 new accounts. I'm going to need more info in regards to this. What type of files are we talking about, and where are they located?
How exactly are the users trying to access them? Have you shared the content with them, or do they access your account online?
Keep me posted!