When we went, we took a taxi - given the cost of Atlantis, the taxi was quite cheap (I think at most $20 each way, for 4 of us). Even if you used 3 taxis, you're looking at less than $15 per person round trip, which is not so bad compared to the cost of the park. I've never heard of free transportation.
If cost is an issue, consider the Comfort Inn option (where you book a room at the Comfort Suites so that you get access to the Atlantis water park). I am not sure if it's more cost effective for 9 of you (not sure how many rooms you'd need, though, so maybe not as much of a savings for you). It was a significant savings for us, though (at the time I went, Atlantis did not have day passes of their own, except through tours, which were more expensive). It was just a little more of a hassle than going straight to Atlantis would've been, but it was not bad at all.