From what i found out, if you play in multiplayer and go down, while not able to come back alone, other players can help you up by clicking on you. Works only as long as you are not dead, means the red bar is not completely gone empty.
Takes not long to res others but you are locked from moving and can take damage while doing so, so better clean up area before you can res someone.
The message that you can't res while other player is in combat is confusing though, i would strongly suggest to explain the difference from singleplayer mode somewhere.