Grunts (Uggnoy) have masks because the only air they can breathe is methane. The masks supply them with it.
they live
I am not currently aware of any Halo games where you can play as grunts
No, they breath methane gas which is why they are (almost) always shown with. The triangular tanks on their backs and masks on their faces, they would need that ,at least, to survive in vacuum.
The covenant is an alien species in halo including grunts and elites.
yes they do
You can only do it on HALO 1 on the Computer with mods.
grunts, jakkels, brutes and elites.
Magnum
By earning achievements.
In halo 2 the hunters, grunts, and elites are technically sided with the arbiter. The arbiter later sides with master chief and around that time the elites decide to not follow the other alien brethren so the hunters and grunts no longer side with you. In the end once all the prophets are dead, the elites return to their home-world so they may once again be a part of their brethren.
You can't it is impossible, anyone who told you that is trying to trick you
Sadly no, the creators of Halo 3 felt that if you had hunters and grunts as allies(the ones that sided with the Elites) and enemies(the ones that sided with the Prophets and Brutes) it would confuse the player. But some still stayed sided with the Elites, but you just don't see them in Halo 3, you see them in Halo 2 during the Covenant Civil War.