I think that we should be able to double msg like /msg 1000101 1000101 [stuff] so that you can convo with more than 1 person. this would really help as lazy people like me can be bothered doing /msg [name] every time
You do realize that, if you want to continue the conversation with both members you were earlier conversing with, you would have to re-type the /msg command anyway, this time including not one, but two players, right? That only applies when another player, who is not taking part in your conversation, decides to message you and thereby interrupts your earlier conversation, of course. Still, you probably get the point; it's a bigger effort in the end.
Oh, and in addition to that: The more people you add to your /msg, the shorter and more limited your actual message will be.
The only problem with that would be that the availability of a player when needed, provided he is in a conversation, suffers. There is a pretty decent command for ignoring players if necessary, being /ignore. Though automatically ignoring incoming messages, from anybody, wouldn't do any good. But again, that is only my opinion.
Here's the problem, the console will have no way of telling if you want to message 2 players or if you want to message 1 player with the message of a player name in front. Plus, you'd have to completely rewrite the plug-in to do that, it may be a bit too hard seeing we don't have an active developer.
The server is using the basic Essentials messaging. It would be possible to slightly modify the code to allow multiple recipients to be added to a message. That is, of course, if the Essentials source code would be publicly available.
You could just use a plugin that allows you to invite multiple players to a "party." You could then do /pchat [message] and anyone in the party will see it. It's little more simple than messaging multiple people.
We had something much easier than this, chat channels. You could create a locked channel (with a password) and chat with just your friends. Here's the immediate downside. It was rarely ever used, and most people used it to advertise, break our chat rules or spam the ever loving crap out of people. These issues would be more prevalent with this suggestion, also as someone stated the more people you add to your /msg the less of a message you could send.
Talk to them on forums...? But as Pittipla said, it's not an option to edit the Essentials code, so the main suggestion is out, but the party idea is still plausible.