The asari were the first race to discover the citadel soon after learning to use the mass relaysthey were joined shortly by the salarians and soon after the volusas other races were contacted by the asari or discovered the station independently they gained embassies on the citadel and came under the jurisdiction of the growing council eventually comprised of the asari the salarians.
Mass effect citadel tower.
A misunderstanding leads to hostile first contact and the unsc is drawn into war with the council shortly after the insurrections height.