In the corner of the bar, a group of angry ranters were loudly complaining about the terrible service.

ranters 是动词 rant 加后缀 -er 派生的名词，rant 意思是"咆哮、怒骂"，-er 表示"做...的人"，所以 ranters 指"大声抱怨的人"或"咆哮者"。