Nope, single player only.
Although, I am not sure, there may be a mod that has some sort of online multiplayer implications (I think trade wise, but not colony versus colony)
I actually rage quit Rimworld a few weeks ago, or I'd be well in excess of 200 hours.
The reason? I had a raid, and they decided to attack a wall instead of the usual way they go. So I moved my colonists to them to get a good shot - there was only two of them so I thought "just go as a group and gun them down".
Obviously I didn't check what weapons the raiders had. Big mistake. One threw a grenade, killed eight colonists at once. Pure rage.
