The megaphone appears to do nothing. First Club member completes his/her part, an announcement automatically goes out to Chat. Anyone else completing, no announcement. If you try the megaphone, you get a reply that someone else already sent notification.
I believe it’s to let you know that you’ve reached the max (yourself) for the quest. For example, if it says Win 450 Games and at least 18 members needed to complete, then each member has to win 25 games. When the mega phone comes up, it let’s you know you’ve completed the 25 games yourself
We have started something in my club, after the first person sends the automatic message, we all post. Maxed. on the message board
we do that too! post when we have maxed.