Set up the trigger as large as you want it, then hit F2 and group the trigger to the rifle squad - select "whole group" from the pulldown list.
In "on activation," type
squadleader sidechat "In position."
, where squadleader is the group leader. Conversely you can put "group1 = group this" in the leader's init field and then substitute group1 for squadleader. This should have the leader say it no matter who the leader is - useful for those times when the leader could be killed.
That's the simple way - if you have an actual audio message in your \Sound folder, you can use commands like "say" or "sideradio" to make the guy play the appropriate radio message. The setup for those commands is the same, though you'll need a stringtable.csv file to store the text of the message - sideradio and say only use the title of the sound file in the command.
There are some sound tutes in the editors depot - check em out