<<<< "Messages", but "Posts by members"
What I've seen in Groups.io (I mean the interface, not user content) is that /message/ is the noun, and /post/ is the action. >>>>>Ok, I could accept that - except that gio uses message and post interchangeably,
Posts by members is a noun (not an action - ie, verb) some number of (1 to ?) messages displayed.
Now, if the screen read "messages that members have posted" that would be a different story.
A message is a noun, ie., one.
Pick one, and stay with it!