Ambient occlusion render passes are usually added to a comp with a Multiply composite mode, but Stefan Minning has found demand, features, and improved render speed enough to share another After Effects filter, Occlusionist.
The filter should be available in a few weeks when render times are "more on par with what After Effects users are comfortable with" (see Comment)
Here's a few words from Stefan (also the maker of the AE filter Normality):
"Ambient occlusion is an approximative global illumination method that checks to see whether any given point on a surface is occluded by other surfaces. ... It’s an incredibly useful tool to give your renders more depth and a more realistic appearance.
The Occlusionist requires a depth and a normal pass to do its thing. Once you’ve gotten those rendered out of your 3D program of choice it’s pretty straightforward to create ambient occlusion effects: Apply the Occlusionist plug-in, calibrate your normal- and depth pass and there you go — clean (or noisy, as you wish) occlusion effects right in After Effects."