With a party member, can I now use my medpack on somebody else to heal them?

I don't see any reason that should be linked to party. That seems like something you should be able to do with a stranger, with an NPC, with a really cute creature you find out in the world. Ugly creatures too, if you feel like they help. I don't see any reason that should be tied into the group system. That seems like a behavior that you would want or not want as part of the med kit itself.