IDEA is a membership association for fitness professionals, ACE is an organization that certifies and trains fitness professionals. There are a few different categories of memberships, depending on whether you are a studio owner, a trainer, and so on. If you go to the top of the ideafit.com page and click on membership or call them at 1 800 999 4332 you can see the options.
It is certainly a good way to keep abreast of trends in the industry, among other things.
They are separate organizations. You can join with the link:
http://www.ideafit.com/fitness-membership. I believe it is well worth the fee.