Dread experience and how it works

I played a game last weekend where I was able to spam a bunch of chronuses... maybe four at a time, or something. 

They chronuses weren't necessarily grouped into an army (can you even do this with dreads?), but they were definitely in the same area fighting.

And I noticed they were all leveling at pretty much the same time. 

So this has me wondering how experience is being handled.

Are the dreads simply getting experience when they are in the proximity of troops that are being destroyed?

Or do the dreads actually need to be doing the destroying themselves?

Or is it sufficient for the dreads to be in the same army as the troops that are doing the destroying?

Or is it based on damage, rather than kills? 

Or maybe some combo of the above?

In this case, I don't think there were enough troops being destroyed to level all of my dreadnaughts at once. 

So... could they have been sharing the experience or something?

Even though they weren't in the same army?