You can join anytime...
However, there are no pro-rated discounts, as the membership is not calendar year annual.
So, if you join June 1st, for example, your membership is renewed on the June 1st timeframe each year. You're not tied to the calendar year.
Unless you're interested in go-karting, not much is happening (outside of monthly meetings) until April.
I'm not really an asshole, I just play one on the Internet