Can multiple accounts use same Dropbox, with access control
Since you came here to ask this, I'll volunteer that a Mac with OS X and Server configured would host files exactly as you wish. Some users could have read/write access and others read only. You could further restrict shares, folders or files with ACL so that arbitrary files are hidden from all but users that are supposed to see the files.

You might also be able to get dropbox group software or Microsoft skydive to do similar, but we're not the best place for that support or answer.