Here is the sentence:
Before the superclub boom that happened either side of the millennium, DJs started out as relatively anonymous record changers, with a handful known only to committed dance acolytes.
I would like to know if the superclub here is a noun or an adjective.
P.S. My friend told me that the noun didn`t use plurally when it is used as an attribute, he gave an example "computer magazine". So he thinks that superclub is a noun in this sentence. But I think that this is an adjective. Who of us is right?
If my friend is right, could you advise me a book where I can read the rule he told me?
Thanks in advance,