As Mizudori wrote, you need to make functions to detect the button clicks.
Make as much functions as much buttons you have in GUI.
Your GUI has 4 buttons so make four such functions.
Example :
function onClickBtn (button , state)
if button =="left" and state=="up" then
--function to put player in desired team
end
end
addEventHandler ( "onClientGUIClick" , button_name , onClickBtn , false)
And also I would recommend you to use dxText instead of read-only memo. It makes the GUI look more attractive.