OK so far (03/07/2013) I have escalated this to a MS support ticket. We have removed/reinstalled Lync and the problem persists. We removed MAC's Office 2011 and re-installed from original installation disks. Before any updates were applied, we tried Online Meeting button. Still did work, same behavior. (btw, MS tech performed a remote desktop session so he could see all versions and see the problem and try a few variations of conferencing. Nothing obvious came out of this, this seems to be a new issue).
So at this point I'm wondering how we get an older copy of Lync. All of our Users have the latest version (check for updates often) and it seems that they are all using the most recent version (which could be a bug in that version). So I've asked (no answer yet) Microsoft Tech for an older version (URI) so we can see at what point (which version) the problem started (we have been on Office 365 since late December 2012 and have had 2-3 Lync client updates on the MACs during that time - most issues seem to point to a period of time when the User's last updated their Lync client). But again, the older version does not seem to be available so that the user can reinstall that previous version.