You can see in your co-op when your members were last online.

You can find the display for this in the overview of your co-op. In front of each player's name there is now a dot indicating the last activity. There are 5 colors to show you the status.

Light Green - This means that the player is currently online.

Dark Green - The player was online today.

Yellow - The player has not been online for up to 48 hours.

Orange - The player has not been in the game for up to seven days.

Red - The player has unfortunately not been in the game for over 7 days.