I am not sure what you were trying to do. If you want particles to come together to form a shape than you need to start with that shape and explod it then pre-compose and use Time remapping to reverse the clip.
Yes, something like that! how can i do that?
First off , you need to create a perfect mask of what you are trying to form.
For example if you are dealing with text you can go to > layer > auto trace " not recommended but its saves you a lot of time instead of masking it yourself "
Then open up the mask propriety and copy the mask path and paste it in a point light position.
Open up particular change the emitter type to light and make sure that is your light name is same as the light name that is set in particular and by default it is Emitter.
The particular set up is not fixed , but those parameters is what I used to create a thin line :
velocity = 0
emitter size x = 0
************** y = 0
************** z = 0
particles per second = increase it if your faced a spacing issues
particles/sec modifier = none
If you didn't understand what I said , tell me and I will try to create a screen shots for your or a video screen capture .