google tries very hard to leave the impression that you must have a gmail/google/google+ (not sure, haven't had the misfortune of doing this for some time) account to subscribe.
You can subscribe via email without any google account by sending a message to <groupname>+subscribe @ googlegroups.com, then it acts just like any other mailing list.
so you were misled and confused by google into thinking you need a google account, if only for a while. they try hard to manipulate people. do you see the problem now? yes, you can fight the stream of lies and misdirections, or you can say "thanks, but no, thanks".