Someone can be convicted of rape on the basis of joint enterprise - the roles can be different, ie someone could act as lookout, someone else could help restrain the victim etc. In very basic terms, the rule about joint enterprise is that someone's role invovlement has to add something to the crime in some way. So someone can be convicted of a gang robbery if they are standing there looking intimidating and adding to the menace of the situation with their presence, but they can't be convicted if they were just standing nearby watching. Obviously it is a difficult evidential line to draw.
From the little we know about the role of the two voyeuristic men, they weren't actively involved in the rape itself (they weren't in the room as I understand it) so couldn't be said to be adding anything to the crime - so it would still have happened even if they weren't filming it. They could, in theory, have been charged with voyeurism but I am guessing the evidence pointed more towards them filming the attack because they thought it was funny, rather than for sexual gratification which is required for a voyeurism conviction.
In terms of the split verdict, I posted about this on the AIBU thread. I am guessing that the two sets of video evidence (CCTV from kebab shop and mobile phone footage) showed her condition deteriorating between her meeting McDonald and being raped by Evans. Her loss of consciousness may even have happened between those two events, making it clear cut for the jury to decide Evans could not have reasonably believed she was consenting, but that they weren't sure about McDonald's belief - remember it is the defendant's state of mind/belief that is being considered.
As I said on the other thread, a split verdict is often believed to indicate that a jury has given proper consideration to the burden of proof and the evidential issues. That they reached different conclusions, whether or not you think the decision was correct, certainly suggests to me that they were grappling with the issue of consent, and probably that they were properly directed by the judge. From the little I have heard about the case it seems to me that the co-defendant was very, very lucky not to be convicted, but I suspect there was something fairly substantial in the evidence that separated the two defendants - possibly to do with the fact that Evans attended deliberately and purely to have sex with a woman who had already had sex with someone else.