Hi @19lin, There are a couple of ways of finding messages that you would like to respond to. You need to know at least one of the following:
the Group
the Title of the discussion thread
the @username of the person you wish to respond to

If you know the group, then go to this page https://connect.mayoclinic.org/groups and select the group, then find the discussion.
You can also use the Search (see the magnifying glass above) to type in words from the title or the discussion to find the thread.

Let me know if this helps.