Update, I've opened a ticket and escalated. The ticket is currently stuck at Tier 2/3 and some sort of software patch is needed (not clear as to whether this is a client patch or server patch).
Here is what I learned. When your Tenant is migrated to the '15' revision (what I call the 2013 servers), at some point after (weeks to months) users contacts are migrated to a new feature call Unified Contact Store (exchange and Lync contacts in one place). It is after this migration where the Lync 2011 client s get stuck. It seems that the MAC clients can't connect to this new Unified Store.
We were told that we could request (through MS tech support) to have a User's Contacts migrated back to the old contact storage locations and things would continue to work as they did previous to migration. We did not go this route...
We were also told that a software patch was being tested but I could never get from them whether it was a client patch or server patch. In either case, we were never told it was released.
And here is our latest bit of strangeness... We had 3 Users affected by this and all of them can now see their Lync Contacts on their MAC (happened about 2 weeks ago). But, they still can't create a Group or add new Contacts. Not sure if that was related to a client software update or a server-side update or none of the above.
btw, from the first time I started support request to now has been about 3 months. So our affected Users went a couple of months w/o Lync Contacts on their MAC's. So 3 months into this issue, we're still working on it...