How do I build a circle which serves as a particle attracter using After Effects Trapcode Particular?
I would recommend you use a different plugin, use the shatter plugin.

Follow the steps in this tutorial

http://www.macprovideo.com/hub/after-effects/after-effects-text-that-blows-away-like-sand

This will let you break apart your circle into particles. Then when done the composition, simply pre-comp the layer and use the time/time-reverse layer to let the particles come into a circle.

While it seems a little backwards(literally) to do it this way, I think this is the best way to accomplish what you are trying to do. 

You won't be able to get a particle attractor or other advanced functions like that unless you are working in cinema 4d or other 3 rendering software.