well the script above will work just fine if you know the names of the group/single loons that are likely to be in the vicinity.
if you want this to be truly independant of names, you need a little heavier scripting. one way would be to create triggers for each loon which could join the player. the triggers would need names, and would be grouped with the player. you'd have to create a looping script which centres each trigger on its respective loon. in the 'on activation' field of each trigger, you'd put the addaction command, calling a script which would display the "hey you, join my squad!" message and join the loon in question to the player, delete/remove the trigger and remove the action from the player's menu.
if you're new to scripting, i'd also heartily recommend the tutorials in the editor's depot. they're pretty easy to understand, and will have you on your way in no time.
good luck
EDIT - i'm in a helpful mood today - even though the bastard bus-driver wouldn't stop - so attached is a sample mission for you to look at, containing exactly what you need.