Is there anything that can share screen between more than two people (like e.g. TeamViewer), but is open-source?
At the very minimum, one user should be able to present their desktop, exact copy of what they are seeing, to one or more other users. Giving the remote user control permission would be nice. Chat/voice/file transfers and similar are not needed; I can always use XMPP/Jingle or something like that for those.