They did not join the UNSC. They simply declared a ceasefire. The aided the UNSC, not join them.
But really, you know how Dr. Halsey calls the discovery of a halo ring a game changer on the level of the conical bullet? The entire reason Halo is that is because it caused the elites to leave the covenant.
The elite split was the single most important and game changing result of finding halo. Without that happening the covenant would have "won".
[Edited on 05.29.2011 9:13 AM PDT]