You should speak to Terranik on here, she's just accross the other side of the river and I'd guess would be more than happy to meet other MTB girls.
Also if you want to know anything about the area in regards to stores, cyclepaths, training rides, road cycling or anything else like that ask me as I've been riding around this place for too long now.
For clubs, WSMTB would be the "local" club and I'd reccomend them, but MWMTB would be just as appropriate. For riding, Manly Dam, Blue Mountains down south (RNP) are your best bets. If you want to do cruisey road distances for fitness' sake then there's a few good quiet (almost car-free) loops on the cycleways around Rhodes, Silverwater and Olympic park.