To resurrect an ally you will need to locate their Soul Heart which is located in their chest armor slot. To acquire their heart you will need to remove their chest armor piece. Equip their soul heart to you utility bar then proceed to look for an Altar of Sacrifice. After locating a Altar of Sacrifice, hold your team mates soul heart in your and press interact button. This initiate a channel action that will resurrect your team mate when the bar is full. Beware that cost of resurrecting your teammate is some health and they will be resurrected with minimal health unless you have a cleric that has Requiem perk. Being hit or moved will cancel this action.