I managed to buy the game in the sale and am having a great time so far. My question is when is a good time to start multiplayer? Should I wait until I finish the campaign or is it ok to join once I get the basics down?

MP and SP are completely different games. It certainly couldn't hurt to learn the basic controls. If you want to practice for MP you want to avoid pausing, but in SP this makes it nearly impossible to manage your henchies. AFAIC just get though the tutorial and you're ready for MP.

Thanks for that. Didn't realise they were completely unconnected.

I wouldn't say they are unconnected so much as different. At the risk of picking nits the controls are the same, but the game play is completely different. Not being able to pause changes the pacing of the game and makes your hot-key configuration critical. I've seen experienced players choke hard in MP because they had to unlearn so many bad habits they picked up in SP. MP also tends to introduce new elements like resting restrictions that SP players are not accustomed to.