Since the basic interaction doesn't have a function between co-op players, it could instead display the survivor's current health, stamina, injuries, etc. (basically all the info surrounding the mini-map).
For NPCs you approach them and click a button to start a conversation. With other players, pushing the same button would instead display their current condition over their heads for a few seconds.
This could provide veteran players more information so they can better support new players who may be unfamiliar with the health/stamina system.
Make it optional of course, another toggle to be added to the HUD or Multiplayer section of the menu.