[Frostmourne27]

AFAIK:
2 slaves: +1 to all paths
4 slaves: +2 to all paths
8 slaves: +3 to all paths
etc

Fatigue is calculated like this I think. Pretend you have a communion of A High Priest of the Sun (F2B3) and 4 Moon Priests (S2B2). High Priest is the master (remember than sabath and communion are interchagable). High priest is now F4B5, and Moon Priests are S4B4. High priest of the sun casts Spell XXX, which requires blood 4. Now the math get really weird: each moon priest get 1/5 of the fatigue to that would be incured by casting this spell with blood 4. The High Priest of the Sun gets 1/5 of the fatigue that a blood 5 mage would get when casting this blood 4 spell. Also note that the fatigue sharing is not optional, and that fatigue over 200 turns into health damage. You can kill communion slaves if you are not carefull. AFAIK, fatigue is only from master to slaves, wheras buffs get shared both ways. (that is, when a communion slave casts a spell, his master doesn't get fatigue - the slave gets it all.)