Is there an addon that will let you know who is in a group by clicking on one of the party ?

Deep down I think I know the answer, but I said I would try to find out.

Plus it's a way of saying Hi! ....... Hi!

No.  The WoW API does not provide a way to find out information about any party other than your own.

You could do it if you and the player you were clicking both had the addon, but that would limit you to checking other people with the addon.