"Now I'm really worried and would appreciate some advice."
You weren't worried when all this happening..? At no point in your mind did you think, hm this could POSSIBLY be illegal and you could get in big trouble? Your friend drugged her or not, it was still wrong, legally and morally. You're sick. You realize things like what you did to that girl will change her life? People have this happen to them all the time because of people like you, and they're never the same after..all because some kids like you wanted to 'have a go at her.' You're a disgrace honestly. Oh God don't even get me started on all the possible things she could have gotten from any of your little 'buddies' OR you could have gotten from her. You have no idea if she had HIV/AIDS, syphilis, chlamydia, gonorrhea or any other forms of STDs..and congratulations, now you're positive for it too. :)
But I guess to answer your real question, yes you're all guilty of gang rape and most importantly guilty of ruining someone's life. Have fun. :)