it totally depends on how cold or how warm it will be..... if it will be cold and raining you'll want more on than if its a warm rain. Be sure to layer enough that you can be comfortable when working hard, but not overly chilled if the group stops for demonstrations etc.

cold rain wants -
long finger gloves
wool socks/water proof socks
booties? (not sure how compatible with mt biking booties are...)
warm beanie under helmet
brimmed hat under helmet to keep rain out of eyes (or helmet with visor?)
torso base layer
vented or soft shell rain jacket
tights or at least knee warmers

warm rain wants-
vented or soft shell rain jacket
if very warm, maybe just a vest or nothing
brimmed hat under helmet


take a towel and dry clothes so that you can dry off and change before you have to ride home.