When you say multiple users I assume you mean each with there own desktop login. Not on the same desktop trying to share things. My guess is that who ever is logged on will be in control. Once you put a user in the background it would not respond. But I have not tested it.
Only one user at a time may access his/her Desktop. However, behind the standard Mac Desktop is a standard UNIX environment. Because of this, any number of users may access and edit shared files and files stored under their account name. Also, command line [telnet] applications and remotely-hosted X11 UNIX applications will run without issue.
Currently iCloud files are locally saved in each user's Library folder, so they are not shared and in fact remain private to each user. (This is not new to Yosemite.)
And it is possible for multiple users to access their desktops simultaneously if the second (or third or ...) user is accessing using Screen Sharing from another computer. (This is a new feature since Mavericks.) This doesn't cause any problems for the reason cited in the first sentence.