From Halopedia, the Halo wiki
Company commander is a billet position in the UNSC Army and Marine Corps. The company commander serves as the commander of a company. This position is usually held by a Captain.
During the Battle of Installation 04, the battalion of ODSTs assigned to the UNSC Pillar of Autumn was running low on officers, so First Lieutenant Melissa McKay was given command of B Company while she simultaneously led 3rd Platoon.