Basically, the way the handle the co-op/invasion/multiplayer means that it's incredibly hard to play with specific dudes. Not only that, it's also harder to find anyone to invade/co-op with because you could end up in a server with no one in it, in a server where everyone is of completely different levels or where everyone is in a different area, in short, they kinda broke one of the most awesome components the game had.
It's still possible to co-op with other dudes, but you have to be very patient or try/pray to get in a server where dudes are actually close to your level or in the same area as you are.
I've noticed it while playing, sometimes there are a lot of messages everywhere, sometimes there are no messages anywhere, same with summoning signs, before one of the bosses I saw a bunch of soul signs and when I returned (got killed the first time ) there were 0 signs anywhere.