Lock it up.
Send it out.
Hide a message, picture or link behind a code or a riddle. Whoever cracks it first sees it, the rest are too late.
Locked.
Send this link to the group chat. First one to crack it wins.
Open the link yourself to see whether anyone's cracked it yet. You won't be able to claim it.
On a roll?
Make another for a different group chat, or a harder one for this one.