Ring groups are not the same as a follow me. Different functionality. So I expected it to work.
But, having never been asked the question before, I set it up and tested.
I added my phone to a new ring group.
I then activated Mobiltiy.
Rang the ring group - my cell phone rang.
I also then tested with a normal hunt gruop. Works the same. In general, if you can make the prime line ring, your mobility should work.
So your answer is, works great. You could add more phones to the ring group and ring several phones at the same time. Your limit becomes how many Mobiltiy lines and trunk lines you have.
Ralph