Touch to get the object's Group Key.

default
{
    state_entry()
    {
       llSetText("Touch to learn\nGroup Key", <1,1,1>, 1);
    }

    touch_start(integer total_number)
    {
        list a = llGetObjectDetails(llGetKey(), [OBJECT_GROUP]);
        string b = llList2String(a,0);
        if (b == "00000000-0000-0000-0000-000000000000")
        {
            llWhisper(0, "Please set the Group for this object in EDIT under the GENERAL tab first.");
        }
        else
        {
            llWhisper(0, "This object's Group Key is: " + b);
        }
    }
}