Okay, finally a few hours I figured out how it works. But thank you all for your time and replies.
function sendGroupMessage(msg)
for index, player in ipairs (getElementsByType("player")) do
if exports["s_groups"]:isPlayerInGroup(player, 1) then
outputChatBox("#F9BF3B[Police]#ffffff " .. msg, player, 255, 255, 255, true)
end
end
end
addEvent("sendGroupMessage", true)
addEventHandler("sendGroupMessage", getRootElement(), sendGroupMessage)