You don't need the game to actually join, although it helps. Part of being in the game is the roleplaying, spamming, and discussions. I'm sure that if you're willing to write some roleplaying or stories, any of the factions will be more than happy to let you in.
Even if you not willing to do that kind of writing as long as you partipate in some of the discussions, then you'll make a good new member.
Some of the hive's members don't even have SMAC/ SMAX.
And if you are lucky, joining in group MSN conversations. Although it has found that gathering of prolific spammers writers tends to generate crazy conversations.