In combat, the health bar on each character portrait doesn't seem to list raw HP values, it's just a colored bar. I'd like to be able to see exactly how much health each character has, so I can determine when using a medkit will have the most effect, for example. I'd also like to be able to do so out of combat, so I know if I need to heal up before a fight.
Go to the character sheet, then click the arrows below their portrait and you will see other characters in your group.
To heal them, change to the inventory screen, select a medkit and use it. You can use anyone's medkit to heal anyone.