They can but they must not be accessing the same .manager file at the same time. This will make Dropbox to create conflicting copies of Manager files.
If there are multiple users, it’s better to switch to cloud or server edition.
As for “Users” section under “Settings” tab, this is for cloud edition and server edition only but since list of users and permissions are going to be stored in .manager file itself, user accounts can be modified on desktop edition too. This is useful when administrator forgets their password and can’t log in through server edition. They can open their .manager file in desktop edition to change their password.