Well, if all you ask for is hey talk to me, I'm not sure what you'll get in reponse. If people aren't on your boards by now I doubt they'd get on board at this point. But if you send a PM and sum up your currrent strategy and how they fit in and ask "how soon can you get an army to X" or "could you focus on Caelum because he's really annoying and we'll defend your northern front" or something like that, you could keep them updated that way.
And if all else fails just fill their land with scouts like we do
