It can be done
Three quarters of our guild are evil, with about a quarter goodie goodies !!
It can be done, and infact, don't be afraid to drag your guildies into some of the evil tasks with you, because the only way they lose faction for goods is if they turn the evil faction crystal in ....
So in an XP grind, you could do 3 good mission with them (and you don't turn the crystal in) and then 3 evil missions for you (and the group not turn in the crystal) ....
I am quite pleased with the way SoE thought thru the DoN progression quests, specifically how anybody in the guild can chose which side to bat for without penalty to others !!! (If only Velious was designed like that!!!!!!!) ...
Good luck and keep your evil ways, I will see you on the Dark side shortly
Tek