Is there a difference in your likelihood of successfully sneaking if you have a follower with you? It would seem like two people would be easier to detect than one, but I can't find anything in the help section confirming this.

From what I can tell it depends on the follower and your relative sneak skill compared to theirs. Some followers sneak pretty well. Others do not. And most of them will run into battle too soon, but that doesn't always mean that the bad guys will notice you. I have gotten the sneak bonus a few times when sneaking up on a bandit that was actively engaged with my follower. You might try the UESP.

The book say's that followers will sneak and that you should choose a follower with good sneak skills if that's what you are doing. Also though that they won't find cover on their own so if sneaking is isential you should leave them behind. I did a quick look on USEP and didn't find a difinitive answer, but it was a quick look.

Followers are a great way to fine tune your Sneak.

If you want to be Sneakier, then take no follower or take a follower that is Sneaky.

If you leveled up Sneaky skills too much and you find too much of the challenge is gone, take a follower that wears heavy armor boots. With Lydia clomping along behind me, I can only sneak into the room, not up to an enemy. Plus, 50% of the time, they notice her and not me, so I can give stealth hits while the enemy is engaged with Lydia.

I'm an archer... if I have a non-stealthy follower, is it best to tell them to stay put, sneak up and find who I'm attacking, sneak attack, and when the enemy charges, run back to the follower so they can engage them in melee?

They definitely affect it. For super stealthy missions, I always left my follower at the entrance and went alone. For normal dungeon delving, a pair of Muffle enchanted boots and/or Fortify Sneak equipment helps a lot.