How do I create attracting and combining enemies?

I'm having quite a difficult time creating this enemy, actually I have no idea where to start!

With this enemy, there has to be two of them to actually start taking effect. Two individuals are attracted to each other and create a bond. Then more come and then more, and soon there is an entire grid of the enemy.

Here's an image of my idea.

https://www.dropbox.com/s/d2zhudvpsb0kv ... y.png?dl=0