I voted "Nobody is to blame", because out of your group no one is. But I think it is obviously that the attacker is the guilty one here.
You did your deed by warning them about the bad people who might be around and offered them a ride home, they didn't want to listen, ridiculed you and went in the knowledge that bad people might be around (this is _not_ victim blaming, you pointed out that there are bad humans around and offered them help).
Now, for the incident, I don't think they are to blame at all, even though you warned them, since they were drunk, however that they now try to shift the blame entirely on you, that is just wrong and you should stop hanging around with those people. I know that is easier said than done, but seriously, do it. Don't go out with the group if you know they are around and if you happen to be somewhere where they are too, ignore them. If somebody asks tell them the story like you told it to us.
As multiple people pointed out already: This is not victim-blaming or slut shaming or any of those other things. I believe that everybody should be able to wear what they want, when they want, where they want. Also people should be allowed to do all those things intoxicated and not be in danger. But sadly there are dumb people out there, who will take any excuse to attack somebody ('she asked for it', yaddy yadda), but those people would attack anybody walking down that road (or park, or whatever), no matter what they were wearing or how drunk they were. Stupid people are stupid and when you know that dumb, agressive people are around and somebody offers help, it might be helpful to take that help. It is important though that we, as a society don't say that anybody would have asked for this (apart from the attacker obviously, who went to the toilet, when human decency was tought), and put the blame where it belongs: to the attacker and the attacker alone.
Again, nobody but the attacker is to blame here, and I hope they get cought and brought to justice and that the whole situation resolves itself easy for you, OP.