For the 1st question, I don't belong on any informal groups since I am a freelancer and a student at the same time, so I am not in a formal workplace setting. But before, I have experienced being in an informal group when working in a call center I just forgot the name.
To the 2nd question, informal groups are counter groups to the main organization or company. Meaning that they intend to counteract the coercive tendencies of the company. Informal groups' role is to fulfill the intention's of the members in it or to protect their self-set rights. Usually, the norms of informal groups produce the feeling of "resistance" towards the company. In some extremes, the employees might get rebellious.
In the past informal group that I joined, We didn't feel very resistant since the informal group wasn't very aggressive and its goals were still a bit inclined towards the goals of the company. In fact, it promoted camaraderie and friendship between its members.
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