yay! congrats, thats great you can put together two teams of just girls! It will be a blast!
Posts above give solid advice on passing - don't sweat it - just ride hard, and listen for bikes and verbal signals from behind. Don't feel pressured and swerve into a tree, stump, branch, root system, poison ivy/oak thicket, briar patch, prickley pear, barrel cactus, yucca, joshusa tree, manzanita, stream, deep mud ( shallow is fun!!) rock, ledgy trail, or other unsafe places just because someone is suddenly behind you. They are faster, and can afford to wait a few seconds for a good place. I like to say "up here" or "around this bend" to acknowledge them if i'm not pulling over immeadiately. Also...this can be big, especially if you have limited trail visibility in really twisty single track, if you think there may be more than one person beind, you could ask "how many?" and count as they pass.
Bring your camera, remember to pre-register (if you can), and you might need to buy a USA cycling license. You can usually buy a one day or opt for the year.
Have you ridden the course before? Pre-riding is always a great idea. You might want to discuss the lap length and how many laps you want to ride consecutivley or not. I have found that its harder for me to ride hard and cycle between riding for 45 and then resting for 45 while my partner rode. I find it easier (and faster) to just keep riding. Just something to think about : ) It all depends on the lap length, terrain, weather, etc.
and...stick around for the awards and any swag that is being handed out : ) Bring enough food to bribe someone if something happens to a bike beyone your fixing abilities.