So I watched the first few seconds of vimeo.com/55250128 by Hammer Chen and immediately pressed the back button because I wanted to try and figure it out. I'm trying to come to grips with TP as I'm used to Particle Flow and new to Thinking Particles.
In his setup he used a box I think to activate the particles to move into the group. I used a Distance Threshold Condition to have the same effect. I didn't blackbox it, but you can easily change the distance to have more or less of an "activation" effect.
Credit goes to Hammer Chen as I just reverse engineered his setup.