The Council of Ancients is the Argwylon counterpart to the Wood Elf council and can be filled by 4 Ancient Treeman lords. Wood Elf lords and Dryad heroes cannot sit on the Council of Ancients, but Orion can if the Wood Elves are confederated into Argwlyon.
Offices can have both faction wide and local benefits.
In order to benefit from faction wide positives of creating an office on the Council an Ancient Treeman simply needs to be given the position after the associated building (which can bring a faction wide bonus itself) is built. One offices does allow forming trade agreements with non-Wood Elf factions and a decrease in local corruption. The trade benefit is factionwide and seems to occur no matter where the officeholder is (though-due to geography-is likely initially limited to Bretonnian factions). The local benefit of decreased corruption only occurs in the province the officeholder currently is in.
Ancient of Talrennic: Provides an additional space for 3 more spellsingers and 3 more branchwraiths factionwide (this can nearly double the number of spellcasting heroes allowed in an army).
Ancient of Delliandra: Enables trade with non-Wood Elves (must have path between capitals) factionwide, local +10 reduction in corruption (both vampire and chaos corruption)
Ancient of Threllock: +50% leader ship aura, local +10% income to buildings in area
Ancient of Druthandor: +10% research factionwide, +100 experience for this lord's army
Offices can be seen and lords selected for them by clicking on the "Offices" button on the wheel in the lower left corner of the campaign screen.