In my experience when it comes to setting EQ levels and finding the right sound in a band. It's better to start with one instrument at a time. Since you really can't change the sound of the drums, that should be your first. Then the instrument which is the most familiar to the drums, the bass guitar. If the bass guitar and the drums sound good together by themselves. You'll have a nice backing rhythm sound. It also gives a nice setting for the guitars to blend too. If both guitars are going to be doing lead and rhythm. Set both guitars to rhythm, and blend one at a time first. Don't use too much bass, just enough to blend
lightly with the bass guitar and drums. Pay particular attention to the mid range since that's where the rhythm sound is. Set highs to where it blends well with the mid and bass settings. Then blend the second guitar in the same way. Tweak where it is needed, then have each guitar switch to lead while the other stays on rhythm. Adjust the highs on the lead guitar sounds, until each note is heard clearly through everything else. That should give you a well balanced sound, play around with some songs everyone knows, and adjust if needed. You shouldn't need to adjust too much; if you do, you may need to go back a few steps to correct whatever problems occured. If you use several effects during different songs, mix each effect settings as above.
Harmonizing, 3rd and 6th's are the most common. Octaves are very common as well, although theres not much of a harmony sound. If someone has an idea, have them play through it and have someone else put their own thing to it. Communication is the thing. Brainstorm ideas until you got something everyone likes.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.