Say (in your indoor voice): “If you can hear my voice, clap once [and then clap your hands once]. If you can hear my voice, clap twice [and then clap your hands twice].”
You will usually have enough people join in after the first and second clap that the rest of the talkers will realize something’s going on. By the time you get to “clap three times” they should be with you. If not, you may want to start over with “clap once.”
Thanks Lisa Nelson for this great attention getter!