Yea, like manekemaan said something like this exists - the basics only though.
Battle Time allows you to play SP campaign however you want, then before a battle it allows you to save the game.
Using that save file it then creates a server right where the battle is going to go down - and friends can join that server and choose their character from your companions.
What this is: Coop Gameplay in Battles, Fun slaughtering enemies with friends, messing with Infantry :b
What this lacks: Coop Gameplay everywhere as suggested earlier. To be honest I believe it would be possible to simply create a player and the game thinks it's a lord, only it can control itself. Maybe that'd work... not sure though.
Anyways guys for now you can try out Battletime :b works great and it really is a lot of fun

At the Moment it only works for Warband 1.143 but there are links to download it on the Battle Time site.
http://forums.taleworlds.com/index.php?topic=150827.0
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link to the Battle Time mod on TaleWorlds Forums