I, on the other hand, wouldn't put a comma, because your sentence is short and its structure is immediately apparent. I think the sentence is entirely correct but rather strange: if you are the best man, obviously you are attending the wedding. The as is odd; it's a bit like saying I'm eating breakfast as I am a human being. I'd pro
Well ,it's grammatically right,but it is quite rare in daily life.Cause the reason given seems unrelevant with your action.It is better to say,I am attending my best friend's wedding.I am his best man.Put them into to independent sentences,the meaning will be mroe coherent.